Create yourself a jig, then cut with siding shears or saw. Remember to wear a facemask if cutting with a saw.

2 3/8 coil nails or I have used roofing nails.

Would actually increase the need for caulking since your suggestion leaves a very large gap between siding and trim, easy for rain driven snow/rain to get under/behind the siding. And, with your idea, you'd still have to caulk between the end of the siding and the 1/8" space where it comes up to the framing.

it's not what hardie recommends. and if you didn't caulk it you'd create lots of cavities for bugs and moisture

Preferably metal sheers specifically for Hardie board siding. You can also use skill saw or chop saw with diamond tip blades but I don't recommend it as you will breathe a lot of dust

To determine the pitch of your Gable measure out 1 foot, then from that Mark measure up straight to the Gable peak. Let's say that measurement is 6 inch then that means you have a 6/12 pitch. The easier way to do this is get a scrap piece of Hardie plank, and trial-an-error till you get the right fit, then you could use that as a template. Then you could flip that template over to use it on the opposite end of the plank to get that angle cut. Oh and the answer to your 8 month ago question it would be a 2 inch nails

You can put a facing trim nail.

Ahm, what are talking about ahm. Ahm, it was a ahm good video.

Um

Yeah, that's what it says. And you can believe that if you like. There just isn't any proof of any of that. Some of us would rather have some actual evidence. I could quote parts of the bible that condone slavery, suppression of women, rape, genocide etc. I don't believe in any of that, myself. But if you want to believe in a book that tells the tales of talking snakes, talking donkeys, witchcraft and superstitions it's up to you.