Paragraph 0:  See below.

Paragraph 1:  What you are describing is a hybrid.  If you use a crossover before the amps, you should remove the crossover that is in the speaker.  

Paragraph 2:  Crossovers come both ways, as do amps and preamps.  Many amps and preamps come with both RCA and XLR.

Paragraph 3:  Sure there are but you can also build one yourself.

Paragraph 4:  Sure.

Paragraph 5:  Considering the level of your equipment (no offense intended), you would be better advised to improve the basic components in order to improve the quality than to fiddle with crossovers and biamping.   Get a decent pair of speakers before kluging two mediocre ones.

Paragraph 6:  I wouldn't bother.

Kal

Work on getting some decent equipment before considering rather esoteric subjects like bi-amping.