People with a physical inability to perform full prostration can perform mental prostrations which, as far as I know, are considered as powerful.
I'd say that for someone with a physical inability to do the physical prostrations, mental ones are certainly better than none and would be wholly worthwhile as a Dharma activity for many reasons, but I'd disagree about the equal power of the two, for the following reasons:
When doing physical prostrations, particularly as part of a ngondro, one is simultaneously practicing with not just the mind but also the body and speech; the visualization & contemplation is said to have the effect of purifying obscurations and negative karmas accumulated by the mind, the recitation of the verse for refuge(and perhaps bodhicitta) is said to purify obscurations and karmas accumulated through speech, and the prostrations themselves are said to purify obscurations and karma accumulated with the body.
So in one fell swoop, one is purifying the body, speech, and
mind of negativities accumulated throughout many lifetimes, averting their ripening with ill effects to one's future bodies, speech, and mind. It's certain that mental prostrations would also have very good results, but they would be somewhat less complete than actual physical ones.
Everything else you mentioned I'd wholeheartedly agree with, though, especially the part about visualizing multiple emanations of oneself and others prostrating simultaneously.
edit: wanted to add that I've heard of some lamas having their students who are unable to accumulate prostrations to substitute them with circumambulations in their ngondro, which to some degree would fulfill the physical purpose of prostrations.