1) Why are non-human plurals considered feminine?
This is an excellent question and there is a lot of discussion on the issue but at this point it may be too difficult to get into it. Instead focus on memorizing the rule for now and being able to identify non-human plurals wherever you can.
2) Is there only one middle vowel change per verb from past to present tense?
Yes, this is overwhelmingly the case. There are some roots that have more than one possible middle vowel change and the dictionary will be able to identify for you verbs for which that is the case.