The phrase "assumes sitting position" seems to imply that the gismu means "sit down", which it, according to Lojbab, doesn't. (That would be {zutse binxo}.)
The "assumes sitting position" is silly, but it seems as though an understanding of the tense system should make it clear that the gismu wouldn't mean that.
No, the tense system doesn't help to decide. There are many gismu that refer to a transition, for example canci.
BTW, does mo'u jimpe mean "fully understand" or "end of understanding"?
I think it means finish understanding, whatever the natural end of understanding is. fully understand should be something like mulno le ka jimpe, (jmimu'o).
So after mi mu'o jimpe, I no longer understand? Mu'o marks that you are done speaking. This is why people often sign messages with mu'omi'e so-and-so. I assume you meand mo'u jimpe
See also gismu that are hard to decipher.