It's not a temporary transformation, it's permanent. Once you're bitten, you become a vampyr for the duration of the fight. It's 'temporary' because it's game mechanics and the game won't permanently transform your character into something else, because I imagine Blizzard would be pissed if their characters suddenly became san'layn, having to bite people every now and then otherwise you become uncontrollable and hostile to all around you, forever, unless you delete the character and start another one, and never fight against the Blood Queen.
Or imagine permanently becoming an abomination because you fought against Professor Putrice and was chosen to be the one to become the abomination.
Yes to all three. These are my top three races I'd want in the game (with Arakkoa being fourth). For he Horde obviously. The Alliance can gave Kobodls or whatever. Obviously the winged models of the Man'ari and San'layn wouldn't be viable for players, but they'd be fitting for the racial leaders. Some Doommaiden for the Man'ari and a new Blood Princess for the San'layn. Bonus points if San'layn have customization options that make them look like a Dark Ranger.
Nothing in actual San'layn lore suggests that they are capable of creating more san'layn by biting people. Lana'thel's ability was either for flavor for the fight or it was a temporary transformation that wore off afterwards since the San'layn have new powers that were granted to them by Frostmourne.
The vrykul in Stormheim is not the same as the san'layn. San'layn were created by Frostmourne, as I said. The vrykul was inflicted with a supernatural blood curse that also prevented him from leaving that area. Yeah the vrykul was more like a traditional vampyr but the only thing san'layn has in common with traditional vampires is their thirst for blood. Other than that, they're basically just a bunch of blood elf death knights.
"Most Fascinating! It would appear that we're dealing with our very own vrykul vampyr! All we have encountered have been of blood elven descent, but it has long been theorized that the vrykul have suffered the curse as well. It's too bad he's awake - I would dearly like a subject to study. Oh well, guess there's nothing for it. Take this stake and put it through the old chap's heart. I'm pretty sure that should do the trick."
That's not what the quest says. Says the vrykul was under the same curse as the San'layn.