Some fans completely misunderstand what the Horde is and how it has evolved. The OP is one of those people. First off, they state that the Horde is 'a union based on honor and the fight against the threats of Kalimdor' with which I disagree.
While honor is important to the Horde, it is not put before everything else although some individuals within the Horde prioritize Honor and exhort others to that ideal. Survival is the core concept of the Horde, the coming together of disparate nations under one banner to stand against a world that hates them.
While an Undead Warchief gave the dishonorable order to burn Teldrassil, Troll hands lit the fires. Orcs manned the catapults. None of them protested. None refused. If honor was supremely important, why didn't they stop? Why didn't they refuse the orders of the Warchief?
Because they were convinced their survival depended upon following her orders.
And before that, Orcs under Garrosh had committed several atrocities. His loyal Kor'kron turned Orgrimmar into a secretive police state where dissent was stifled.
The vast majority of Orcs living would have grown up in internment camps and what I think drives the modern Orc is the fear of being put back in one if the Alliance ever becomes strong enough to overpower them. The Trolls of the Darkspear have been hunted from one location to the next by the Gurubashi, then the Naga and now the Alliance. The Tauren have only just recently secured for themselves a homeland that was soon placed under siege by the Alliance.
And the drumbeat of racism from the Alliance has been feeding those fears for a generation.
To say that Orcish honor is a hypocrisy is too far, rather, honor for many takes second place to survival if push comes to shove We have seen enough Orcs willing to compromise their honor if the circumstances demanded it to believe it to be an immutable factor in their decision making. Rather, honor is the ideal they strive for, and far too often they fall short. That is not bad writing, that is believable, living beings are not robots of whom each individual is capable of adhering to a social code at all costs. Saurfang is an exceptional paragon of honor BECAUSE he is the exception, the aspirational ideal. If Saurfang's sense of honor was commonplace it wouldn't be as remarkable as it is.
And there is also the fact that Orcish honor is not what you believe it to be anyway. Saurfang happily went along with the plan to attack and occupy Teldrassil after all, despite the fact it would certainly trigger a vast world war. And a huge conspiracy was undertaken to trick the Alliance out of position to facilitate this invasion which Saurfang went along with, even to the point of play acting a fight with Nathanos.
So, in summary, pretending that the original races of the Horde have an innate moral superiority to the races that came after is laughable. What isn't laughable is the profound misunderstanding of what the Horde is because of that belief.
The true core of the Horde is the need to survive and thrive in a world (primarily, the settled members of the Alliance who had the world just the way they liked it and want to make it that way again) that would happily see all of them dead or in chains. Every race within the Horde shares this goal, this desire.
Once you understand that the need to survive is the core of the Horde, and not honor (though the ideal is important for many) you see how the Horde can and does work. How the disparate nations focus on what truly matters, making certain that they will persist, and how that unifies the arcanists of Silvermoon and Suramar, the tribal nations of Durotar and Mulgore, the libertarians of Azshara and the risen dead of Lordaeron. The need to survive and provide a future for your descendants.
Beyond the in game rationale, the out of game fact is a faction consisting of three tribal, monster races with similar themes would be overwhelmingly bland and boring. The multi-cultural, multi-national Horde whose architectural range has now expanded beyond spikes to the elven cities of Suramar and Silvermoon, the gothic Forsaken, the crazy mechanisms of the Goblins and now the gorgeous Aztec-Dinosaur deco of the Zandalari is simply far more interesting visually and thematically.
So no, the Horde should not split into two factions. Nor will it. The Horde is perfectly fine as it is.
I wouldn't mind there being an opt-in renegade/mercenary faction that either side could join with full knowledge they'd be burning all faction and current guild bridges by doing so. Lot of potential there IMO. Kind of like a non-faction, but somehow with it's own storyline. (Bet it'd make the RPers happy as hell too) There's already NPC's that would fit that bill, so it'd totally make sense to have it. It'd of made sense to of had it forever ago.
It's just the balance of power that is messed up, the Forsaken were never meant to take the reins. If they pull an Illidan on Sylvanas everything will be alright.
The undeniable truth is that blizzard has no clue about believable logistics or historical economic develepments. In truth we wouldn't actually get stronger by constantly fighting, that is some saturday morning "love conquers all" kind of rubbish. We spend most of our resources fighting over scraps of land all while never really making progress, both the horde and alliance should be in economical ruin by now. Wow doesn't even do the "war kickstarts industrialization", since all thing tech will always just be relegated to grnomes and goblins, both of which power their contraptions by unicorn farts seemingly, so there isn't any technological advances either, with the exception of "McGuffin war-contraption of the expansion" that is conveniently forgotten later on. Heck all the azerite shit we have at the moment is already built with an expiration date, as apparently it loses it's power once azeroth does no longer "bleed her essence" (as can be seen with Gallywix' scepter).
Still wondering why I play this game.
I'm a Rogue and I also made a spreadsheet for the Order Hall that is updated for BfA.
I always chuckle at the "undead + belf" faction cryers, like the orcs who are very prone to go full nazi if unchecked aren't way closer to the undead than any other faction. The tribal "honor codex" has been a pretense since forever. There is a single orc clan that managed to bei somewhat honorable, but that is one out of what? 8? Then we have goblins who just have no morals what so ever, if you want your evil faction you have it right there.
You're using outdated/wrong information though. Sylvanas wasn't a Blood Elf. She was a High Elf. Both of her sisters are still High Elves, there's no reason to believe Sylvanas would have sided with Kael'thas over them if she had survived. She would most likely be an Alliance hero if she had not been killed by Arthas, same as Alleria and Vereesa.
All that aside, any respect the Blood Elves once had for their former Ranger General has been long gone. Lor'themar does not trust her at all. He makes that very clear in the Siege of Orgrimmar on more than one occasion. When she suggests that she can raise the Blood Elves that have fallen at the Gate of Retribution, Lor'themar threatens her. At the end of the raid, after the cutscene where Vol'jin is named the new Warchief, you can talk to Lor'themar and he hints that Sylvanas is nothing but trouble and will likely mess up the flimsy truce between the Horde and the Alliance.
The portal is there purely for gameplay reasons and Silvermoon City has not gotten any kind of lore update in over 10 years, so it's hardly something you can take seriously. But even if you want to roleplay about it and pretend there's a lore reason, tensions between allies doesn't mean you cut off all contacts with them. The whole point of the Horde is that they are all different groups of people with hardly anything in common except their enemy. They're not meant to be a close-knit community, they are all banding together out of convenience. Disagreements are bound to happen and you just have to put that aside to focus on the common goal.
Simply put: they don't want to deal with the Forsaken but they have to. Lor'themar was willing to ditch the Horde entirely during the Pandaria campaign.
Sadly we have no idea what his current stance on her is. At the siege of lordaeron keep he just follows her orders blindly without any hint as to agreeing or disagreeing with her. He is also the only racial leader for which we have no information what so ever in regards to what he thinks about the current war at all. For baine we know at least that he is being blackmailed by slyvanas, gallywix only wants money, the trolls don't really have a leader right now, the orc one just said "fuck you - I'm out" and the forsaken one is the cause of all the problems to begin with..