It's not just acacia that's different from other trees. Oak trees are more closely related to roses than pines.

Being tree-shaped is advantageous for a wide variety of plants and was independently evolved many many times.

I don't think the vitamins explanation is a good one; whatever it is that the mushroom needs as a vitamin has to be something that is so difficult (evolutionarily speaking) for the mushroom to "learn" to produce itself that growing into a giant mushroom tree is easier, which means it has to be, like, an entirely different category of compound than anything the mushroom produces (nitrogen, for instance, is easy to work with biologically unless you have to break an N-N triple bond, and in that case only a few specialized organisms, mostly bacteria, do that at all; you'd need a similar thing for mushrooms)

The HS explanation is clever. I don't know if it's energetically plausible, but it's certainly clever.