The -masu form stem can be used to continue the compound sentence, as same as the -te form.
e.g.
売り上げも増え、 = 売り上げも増えて、
The former sounds relatively formal/written-wordly, whereas the latter is more colloquial. The one-mora stem い(the -masu form stem of いる) is hardly used for the fomer case, as Glenn-san wrote.
Also, can you shed any light on what form おり is in the original sentence (question 3)? Is this a shortened form of something else too? Or is it OK to end a clause with this combining form of おる (if that's what it is)?
おり is exactly the -masu form stem of おる. This is a kind of archaic expression, so 増えておって is not used for the case. (It sounds like an old people's word.)