How much comparing to another paper is too much?
1) Is it bad to compare to one single paper often in a paper?

Not at all. It happens all the time. If it represents a significant improvement of the paper in question, especially one that opens up new possibilities, you don't have to hide the fact that this is what you have done. The same answer applies to your question 3). Whether or not it is a significant improvement of the paper (and thus the answer to your question 2) is exactly the sort of thing that peer-review is supposed to address. Trust your instincts. Resist the temptation to inflate what you have just to get a larger paper.

As far as the second set of questions go, you could write two papers and submit them simultaneously, the second referencing the first. It isn't uncommon for researchers to cite unpublished but submitted work.