Hi Dave,
Merry Christmas !!
I've actively contested with LOGic for years in ARRL DX, VHF, SS, CQ WW, CQ WPX, etc., etc.. Not an N1MM but works fine, some prefer to use a contest logger and import into LOGic at the end of the contest, I personally don't like that approach plus I have customized/automated LOGic to where it does everything but wash the dishes at the end of the day so anything imported from another logger is just a lot of inevitable and unnecessary futzing/tweaking of hundreds of QSO's. I'm not a big gun contester, regardless, LOGic isn't the reason I'm not in the top ten....hihi !! Anyways, my comments;
1) Correct, LOGic will NOT choke.
2) LOGic shows QSO and Multiplier counts and calculates the score, all displayed in a mini window.
3) All in real time during the contest.
Important to know if the Marathon, 1) allows/includes QSO's from CQWW, and 2) would you be working CQWW AND also making non-contest Marathon QSO's during CQWW.
The CQWW Contest Logform could work and would score based on those rules, LOGic uses a Contest ID (name) and start date (and end date in certain circumstances) to "separate" and score contests. Dennis I'm sure will chime in and may have another solution but if answers above are No/No or Yes/No then seems to me should work fine, if Yes/Yes then hmmmmm, may need to think on that or for sure Dennis to chime in !!
73 de Brad, N8GLS