Does an aron kodesh need to have legs that connect it to the ground? Or can it be mounted onto a wall?
I don’t see how that should be different from an aron built into the wall which is common. The more ancient arons were not freestanding. See here – https://www.jewishvirtuallibrary.org/ark.
The Rema on OH 154:3 does note certain requirements for what can constitute an aron:
הגה: יש אומרים דלא מקרי ארון הקודש אלא אם הוא […]