Heavy Air Mainsail Trim
Most of the advice you have received is correct. However, I would disagree with tightening the backstay. That would only move the center of effort aft, thereby increasing weather helm.
The one additional sail shaping tool you might want to consider is a preventer line from the toe rail forward of the boom attachment up to the boom. By triangulating the boom between the sheet and the preventer you can flatten and stabalize the sail just at the point of luffing. Any puffs that over power you get immediately dumped from the sail yet the boom stays stable. I rig my preventer lines ( I rig port and starboard lines if I am having to tack in through heavy seas) back to the cockpit through snatch blocks attached to strongly backed pad-eyes. It is a simple matter to release the preventer before beginning a tack, and with practice, you will become proficient in cleating off the preventer before trimming the main at just the right place so that when the main is trimmed,the preventer is tight.
A preventer line is a life saver if you are having to jibe your way home. I single-handed a lot, and by controlling the boom with both the mainsheet and preventer, jibing was a nonevent.
Don''t forget to release the topping lift.