1. So would using が in that sentence (日本にはチップの習慣がないので、その点が簡単である) have the same unwanted contrastive effect as は?
その点が簡単である
Emphasis is on その点 (=no custom of tipping), it is simple on this point/aspect.
その点は簡単である
No tipping makes things simple (but there could be other factors which might be complicated - implying there are other points to be considered).
5. This is an explanation of ~たいんですが:
自分の希望を先に話すことによって婉曲表現になる。
I'm assuming this is something about ~たいんですが being a euphemistic expression for speaking of one's wishes, but I have a bunch of questions:
a) What is the purpose of 先に? Could it mean "before" in the sense of "before one's wishes are fulfilled"?
先に = first.
First give the reasoning i.e. why you're asking the question (there actually is the goods you've already bought and want to return) before asking if it is possible to exchange the things.
b) What is the purpose of によって here?
by (means of...)
by telling your wish first
c) Why is になる used (rather than です, for example)?
It is not 婉曲表現 as it is, by it becomes (~になる) by telling your wish first
d) Although the dictionary defines 婉曲表現 as "euphemistic expression", is ~たいんですが really euphemistic? It doesn't seem to me as if it is.
返品したいんです is just a statement and might sound too strong (as if the other party must accept it or you think the other party should accept it).
By saying 返品したいんですが, this が adds your wondering, doubts whether the other party actually finds it acceptable, thus it sounds less presuming.