To the first portion of your quote, if Sylvanas would've waited then the Horde would've been decimated. Thrall was on the ground being beaten down, Vol'jin was fatally wounded, and Baine was badly injured to where he couldn't fight anymore as well. Sylvanas was the only Horde leader left in fighting condition while Genn, Mekkatorque, Jaina, and Varian were all either lightly wounded or were otherwise in good fighting condition. So what you're saying is that the Horde should've sacrificed all of their leaders save for probably Sylvanas for the sake of possibly saving Varian, yeah good joke. That's by no means a reasonable expectation.
Now onto the second portion. The Horde and Alliance forces were working together, but they were by no means a military alliance. This means that they had very little recourse for long communication of battle plans, such as a coordinated retreat. If Sylvanas wanted to coordinate a tactical retreat with Wrynn then they would've needed either some form of long distance communication available which wasn't or the ability to send a messenger to Wrynn either back tracking and crossing a moat of fel lava or jumping down a deadly fall to relay the message. The point is that they were two individual forces working together, and that it's ridiculous to expect such a ragtag alliance to have that level of coordination. And before you say that she could've just shouted down to Wrynn, I would like to remind you that Gul'dan would hear that shout as well which means he could've summoned down that fel reaver preemptively in order to block their path of retreat.
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For Azeroth over Lok'tar Ogar.