let's see, question 1: spondaic ending refers to the 5th foot spondee ending, as you suggest 2nd.
diaresis: no, really every word ends at either a caesura or diaresis by definition; of these the only ones of practical interest are the ones which can show you the 'joins' in homeric verses, i.e. the common starting or ending points of homeric formulae. 2 such points are the caesura and the bucolic diaresis, where you have many formula sets beginning and ending, and you have correption patterns different to all other points in the line.
relationship between caesura and buc diaresis? well, i've noticed from experience that most verses which have a 4th foot caesura also have a bucolic diaresis, just a side point. i'm not sure what kind of relationship there could be between a 3rd foot caesura and the buc diaresis.
for the rest of the questions, if you have 2 possible caesurae in a line, it's not necessarily the case that one is "true" and one is false. the caesura is an explanation by later people of patterns they saw in grk poetry. in that sense identifying the divisions at all is an "artificial" exercise, however in another sense it isn't because, as i said above, it shows you the joins, the starting and ending points, which most often occur due to the way homeric verses were often built out of formulae of particular metrical lengths and positions in the line.
finally you're also right when you say a caesura or diaresis doesn't occur before an enclitic or after ÎºÎ±Î¯: e.g. i don't mark a buc diaresis before Î¼Î¹Î½ in Iliad A 441:
http://www.freewebs.com/mhninaeide/Ilia ... an2005.pdf
the same rule applies in other types of poetry (see e.g. ss 5 and 6 of Sidgwick's Greek Verse Comp for tragic dialogue).