MyBB Hacks

I would like to know ways I can apply security fixes and other enhancements to my copy of MyBB 1.6. I have the last release in the series (1.6.18), but I am not yet ready for 1.8 and beyond. I did so much core edits and template changes over the years. The upgrade process literally messes up everything for me. There are many plugins for 1.6 I can't use on 1.8 I want to keep, but the authors haven't updated them or the 1.8 versions/"alternatives" suck. IMO, 1.8 is less stable for me to use than 1.6 in some areas. hence why I wanna still keep using 1.6 while maintaining stability.

For starters, how do I edit the PM feature to limit quotes to just the last message? You can do that for posts, but not PMs, which is annoying seeing a pyramid of quotes.

Since official mybb.com support won't help me with this question, I am asking my question here. Where do I begin with applying some 1.8 fixes to 1.6? I already know about securing ACP, htaccess, directory privacy, etc. Thanks.

You're pretty much attempting surgery to create some hybrid 1.6/1.8 here, which may work, but probably won't unless you're sure about what you're doing.
You could try to diff some files between the two versions then port code changes across.  Alternatively, you can try searching through MyBB's Git history and apply commits onto your copy of 1.6.  But you may actually find it easier to just port your code to 1.8, rather than port 1.8 to your copy of 1.6.