Is there any info or writeup on the optimal way to gamble?
Are rings and ammys the focus? Or circs?
Does it matter which circ type?
Does the game mode or vendor matter?

Etc. Etc.

depends on what you want to get... I would not go for amulets, since most builds prefer crafted ones with high for. exceptions are of course GM melee leagues with demand for high stat amus...
rare rings are always worth a shot, circs as well (especially since circs can't be crafted)

type of circ does not really matter, these are mostly cosmetic (def is neglectable anyway)

no, only char level. but I suggest hell mode, so you can sell back your failed items to obtain at least some of your gold back