I'll start with # 2 :
question with "equivalent point" always is, than in not-so obvious cases you'll have to come up with a sensible definition of the very meaning all by yourself: "equivalent" to what exactly??
(and the answer has, of course, to be problem related)
..which gives you kind of a "responsible freedom", if you know what I mean
for a simple acid-base titration (or even a reverse titration, like in the original question) this usually is no problem, and the mathematical definition you proposed seems a good one to me.
when more complex situations are posed (like for example in the presence of buffers of some sort or another, or additional pH-active species), in my opinion the situation is too complex to have something like a meaningfull "equivalent point" at all - except you'll define one esp. for the setup given.
so, I think there's no general answer to the question how to handle those complicated cases: depending on the problem, you'll have to come up with a usefull definition of your own
ref #1:
yes, I think we can:
for solution #2 , the only resonable situation would be that it has still some acid capacity left - else additional KOH would be quite meaningless.
Furthermore, looking at the concentrations / volumes involved, there is strong evidence that hence the solution should be on the strong acidic side at this moment
for solution #3 , the substances present (due to my proposal for "equivalence point") should be " dissolved K2SO4 plus (NH4)2SO4 " at approx. equal concentrations ("quick and dirty" calculation from the initial values given)
... which makes it a solution of ammonium, sulfate at approx. equal concentrations with respect to pH active species.
now, ammonium is by a factor of ~ 103 the stronger acid than sulfate is basic, hence the total outcome should be on the acidic side.
being a weak acid nontheless, the result should be on the weakly acidic side, to be more specific.
regards
Ingo