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you have covered all the basics from the looks of it. I suspect carbon build up on the valves causing cold missfires. there are many products that will help with this. sea foam is a good product I use all the time. there are times that nothing will work and the heads need to be removed and the head disassembled to remove the carbon hardened on the valves.
try the sea foam first
that is for the injectors, not the valves.
hopefully it will work for you.
no, the carbon is inside the cylinder head, not accessible from the top end.
how are you making out