Okay I cooked sauce yesterday and it turned out OK for first time. To answer some of your suggestions:
1.) I know most of you had concerns there was too much red onions. Actually, after roasting it together with garlic and peppers it just added a bit of the sweetness to the sauce. I didn't think it was too much. I imagine it would be too much if I added fresh onions to the sauce.
2.) Some people told me that it might be a bit too much of a sugar. A friend suggested me I should use 0.2L of vinegar and lime juice (that is 0.85 cups if you're american). Adding anything less than 100g of sugar (I guess that's 1/5 pounds or 3.5 ounces) would make it too sour in my opinion.
3.) I used around 300g of chillies (carolina reapers, 7 pot jonahs, Jolokia yellows, etc.) but I wouldn't mind if the sauce turned out a bit sharper! It is hot, but nothing too dramatic. It's weird because I used all the strong chillies without any mild ones for the base, yet the sauce turned out I would say 6/10 on the scale of spicyness.
4.) I just used tomato with no other fruit added. I think that was a safe start and now I'm slowly going to progress and experiment with some more flavors. Thinking about going for Peach/mango combination or perhaps Persimmon.
5.) What's the deal with the pH of the sauce? storage? to be honest I didn't measure it. However I cooked it until it boiled, got the flasks up to 100 degrees in the oven and filled the hot flasks with boiling temperature sauce all the way to the top, and seal it tight. That should be good regardless of the pH value ye?
I like the sauce, gonna post a picture soon. If I could just point at one thing that bothers me is the bitter aftertaste of something... I don't know what it is, but I think one of the chillies leaves really strong bitter aftertaste... or do you have any other ideas?