As a suffix, yes.
But there are also words that have -ul as part of the stem rather than as a suffix, such as regulo (rule) or nebulo (fog).
Sort of like how "-er" in English is "person who does something" (singer, reader, ...), but a finger is not something that fings -- here, "er" is simply part of the stem.
Sometimes, it's ambiguous: regulo could be a rule (regul-o) or a person who rules (reg-ul-o).
"Maltrovis" is one of those words which you can get away with using, but only in specific contexts and always as an affectation or joke.
- "Ni trovis la vojon sed tuj poste ni maltrovis gxin." We found our way and immediately unfound it.
"Perdi" and "trovi" don't quite exist on a line so "mal" doesn't quite work.
It's pretty much "ĉar = because, pro = because of".
One stands before a clause with a verb in it; the other stands before a noun.
You can't say (in standard English) "I took an umbrella because the rain", nor "I took an umbrella because of it was raining". Similarly in Esperanto, it's pro la pluvo (because of the rain) but ĉar pluvis (because it was raining).