I messed around with some savestates and resetting at different spots during the credits for all the endings yesterday, and it turns out you get credit for each ending right before the "ENDING (#)" text shows up

So the points where you can reset for each ending: 1 (SOS): "happily ever after" textbox fades 2 (dead at sea): "Never woke up again" textbox fades 3 (100 days duo): "happily ever after" textbox fades 4 (solo boat): Konami all rights reserved credits text fades 5 (duo boat, die): Konami all rights reserved credits text fades 6 (duo boat): Konami all rights reserved credits text fades 7 (100 days solo): "happily ever after" textbox fades 8 (best end): don't reset this one, lmao (but it's basically right after the last wedding dialogue, if you really wanted to for whatever reason)

Basically, as soon as it fades out to set up the ENDING (#) screen is when you can reset, and it's only particularly useful in all endings runs since any single ending run would have the timer stopped by then. It saves a bit over 10 seconds per, although it does feel a little scary to do because you only really get confirmation that you did it right the first skip you do when the "mini 2" text shows up on the menu, and at the last ending when you get the full picture. Highly recommend checking the timings yourself with savestates before putting them into a run :P