Been through this only a couple of months ago
Firstly the A-B-C-D method is quite acceptable as long as one of B,C or D is at least 250nm from A. The rule is pretty clear, but one of the FI where I was doing my commercial training disagreed with my FI, but a straw poll, a call to the FSDO and my DPE all agree, ABCD is fine as long as one is 250nm from start and the total is 300nm +
Secondly, Whopity, - sorry but the FAA interpretations on this are based on common sense not rigorous definition ! There is no definition of 'flight' in the FARs for this purpose as the OP correctly pointed out. If you dig into the 'john lynch' FAA clarifications and other letters of interpretation, actually what day it is or length of stop has no real bearing on the matter. Your A-B-C on day one and C-A on day two is in total only one flight for this particular purpose, for sure. They might argue with you about it if C-A was a month later or you really took liberties, but the final authority on this point is the DPE at the time of entry to your checkride. If he likes your x-c then that is that. If he doesn't, you can ask if you or he can call the FSDO for clarification, there is a hotline for DPE's who are in the middle of a test.
The real moral here is - get to speak with your DPE before you enter the checkride if you have anything at all that is out of the normal for the average american student in that position. My x-c flights for the private were mostly in the UK, I spent 10 mins showing the US DPE on google earth the x-c places in my logbook and he was ok with that. He then pulled me up on the lack of Night x-c DUAL I had, despite a UK night rating and 15 hours or so night solo x-c in the US and I had to do a 100 mile dual x-c before the checkride. Good to find that out before checkride day. On the commercial, I already spoke with the DPE about the 300nm x-c as mine is ABCD and he was fine with that.