Now it is true that due to his hard preaching he is imprisoned in verse 20. I don't think that chronological order can be assumed verse by verse. It seems that Luke is trying to clarify that Jesus was baptized at the time that John was baptizing and not after he was imprisoned. The scripture cannot contradict and based on your other 2 citations, this seems to be the only logical explanation. Question 3 .
Immersion is the only baptism spoken of or described in scripture.
We base this on two facts. This means to dip or immerse. They had another word for pour, ballo. And yet another word for sprinkle, Rhantizo. We see Philip and the eunuch go down into the water and come up out of the water.