To answer your original question, God caused the donkey to speak - gave it the ability to speak (Numbers 22:28), but it was the donkey speaking - not God speaking through the donkey. The serpent is not so clear. Genesis seems to imply it was an actual serpent, but Revelation 12:9 seems to imply it was Satan in serpent's form.
Yes. It actually happened.
Exactly. Miracles of "Biblical proportion" (so to speak) are very rare. Their frequency in the Bible may make it seem as if they happen all the time, but if you count up the number of big miracles in the Bible and compare that to the number of people who have lived on earth and the timespan over which they lived, you'll realize very few people have ever seen such a thing.
With that said, don't think miracles never happen today. There was a study done on miracle claims because people always expect it has decreased in the "scientific" age. What the study found is that isn't true. Claims of miracles are just as frequent now as they ever have been. Of course some of those claims are probably false - or maybe even demonically inspired - but true miracles still happen. It's just that, as in Biblical times, they are a rather rare phenomenon.
Or it could be the Bible is recounting a miracle. I understand people such as yourself who don't believe in miracles have to invent another explanation, but in the case of Balaam's donkey, such an explanation doesn't fit the text at all.
Given the direction this conversation has gone, maybe some further discussion on miracles is warranted - if people are interested.