I'm afraid having Valinde together with all other characters in a game would be a bit too much. The Burning Legion that came through the portal after Smolderas summoned the portal, together with the fel orcs, are more than enough forces to wipe out pretty much every living opposing force that would stand on its way. Not to mention they all do full Chaos damage. So the Guardians will stand their ground and fight the Legion for as long as they could and buy Malfurion time until he reaches the World Tree, but would eventually fall. If you remember the interlude of the Burning Legion coming from the portal after chapter 5 in the early state of the Sentinel campaign, they have literally crushed the Guardians with ease. So, I don't think the loses that both factions sustained would be equal. By the way, there will be no time to sabotage the Guardians, since the Legion instantly launched the attack upon its arrival on Ashenvale grounds. And knowing Kil'jaeden's patience, I'm sure they wouldn't just sit around and wait to get discovered

Since we're talking about the Sentinel storyline, I'm pretty sure the main aim of the campaign would be to unite every reasonable faction that could see the truth in Malfurion's words of surviving the Legion onslaught, unlike the Highborne story, in which Malfurion turned evil and sacrificed it all for power to defeat the Legion. That said, my guess is that the Malfurion Sentinel forces would come across many other factions, including Illidan's Highborne forces, and try to find a convincing way of joining forces. Maybe Tyrande could make a peace between the two devided brothers Stormrage, we can never know. But as you said, if the Makrura faction could join the Sentinels, it's a justified assumption that the Ashenvale creeps could join them too! Just look at the tech-tree: Sentinels have Furbolgs, Guardians have Wildkin, the Dragon Aspects have red/bronze/green Dragons, and that's half of the current creep units under the control of the Night Elves. So, adding the Highborne faction, the Makrura and probably some more factions that we may come across while playing the campaign could make a great impact on a storyline and increase the chances of defeating the Legion. Also, adding all the heroes that have joined the fight against the Legion would make a good final boss fight at the end, since Malfurion stays a normal Night Elf hero instead of becoming a super-strong villain in a Highborne story
