Well…remember the wizards don’t “know” that’s what they are doing. There is some sort of clue or joint dynamic between summoner and summonee regarding appearance…I don’t pretend to understand it.
As far as reshaping, remember most demons can’t shapeshift, at least not for very long/permanently. Not all Greater Demons can do it. I doubt any Major demons can at all. So really only ArchDemons and Demon Princes can and do reliably shape shift.
And trust me, they do. However, for reasons explained elsewhere, most don’t have more than a few base forms. You really have to practice them/use them to be able to maintain the different forms under duress and without expending excess mana and concentration.
consider this: the demon’s first form is their “lowest energy state” it doesn’t take any work to maintain it, it’s a default state. All other states require more energy to maintain. The more you use the other form, the less energy it takes to maintain it, but you really only so much time in a day…
So again, the older the demon, the more forms they are likely to have, I suspect some of the demon princes probably can shift rapidly and in combat, if they choose, but if they can, the only way you’d know about is if they did in the process of killing you…because I’d guess they’d also kill anyone who saw them materializing chainsaws on the ends of their arms….