The beef between the BELFs and NELFs is definitely not one-sided historically though in more recent years, it's more personal for the BELFs. Thalyssra states in the Nightborne unlock chain the only reason she decided on the Horde over the Alliance is because she was put off by how overly cautious Tyrande was with her despite the fact the Nightborne (re)joining their Kaldorei kin would've been the obvious choice. Basically, Tyrande made a bad first impression and it cost the Alliance Suramar and The Nightborne.
As for the Night Elves in northwest Ghostlands. Lorewise, the BELFs joined the Horde before Kael split off from them. Tyrande probably knew by then that some of the BELFs went to Outland and were working with Illidan under Kael'Thas. Naturally, she also know the Scourge sacked Quel'Thalas by then and probably found it suspect the BELFs were more focused on their Magic Addiction at the time than rebuilding their half-destroyed kingdom. The Night Elves probably considered the Blood Elves to be "soft" targets compared to the much stronger Forsaken despite the fact Sylvanas Windrunner is originally from Quel'Thalas. They miscalculated and assumed even with help from the Horde, the Blood Elves would ultimately destroy themselves.
I like to think Lorewise, a mixed detachment of Blood Elves backed by the Forsaken routed the Night Elves in the Ghostlands. The attack plans found there, the Dwarf "Ambassador" you kill outside Silvermoon and the number of Night Elves--don't forget you also find some in Eversong--indicates the Alliance as a whole and not just the Night Elves were looking to establish a permanent foothold in the region. It makes sense the Blood Elves wouldn't be willing to tolerate the Night Elf presence in the Ghostlands after the scouts that were found in Eversong Woods. If the Night Elves were left alone for a bit longer, they probably would have had enough time to build a proper base similar in sophistication to Lunarfall on Draenor. That would have made them harder to remove and more so once the Night Elves got their siege weapons shipped in.
Hypothetically speaking, if said base was still standing when Varian returns for WOTLK he would've sent resources and manpower to reinforce it. Assuming it's still there for Cataclysm, the Blood Elves would be committing a lot of resources to taking it. With help from the Forsaken, they would probably rebuild and strengthen nearby outposts just to deal with the base. Alliance side, they would look to claim as much ground as they can to maintain the base. It's a hypothetical we may never know obviously.
...This.
Fandral is revealed as a traitor in the novel Stormrage, which takes place between WOTLK and Cataclysm. It's revealed he used Mercenaries (us) to secretly bring him regeants (remember that repeatable quest from him?) to keep Malfurion in the Emerald Dream. He turned on the Alliance after his son was killed during the War of Shifting Sands which he was Co-Commander of alongside Varok Saurfang. Malfurion was the biggest threat to his interests so it makes sense he wouldn't want him to be woken up.