Im not sure if my opinion is what you're looking for as I've only cleared about a dozen playfields but I do paint for a living and I spray clear every day.
I never see anyone using a syphon fed gun, gravity fed is the way to go. Personally I prefer sata but I have switched to iwata because the paint transfer efficiency is so much better. Its amazing how little clear these guns use and they spray well at a much lower pressure (as low as 16 lbs). As far as cups we use 3m PPS system instead of plastic or aluminum cups but im not sure thats cost effective for occasional use.

The lph 400 is still a great gun. They have the newer bigger thing out now but someone who doesn’t spray every day will never know the difference.
Not every gun sprays good at the same pressure, I don’t feel like 16psi is the sweet spot but it works ok. You probably use a little less air at lower pressure but even more noticeable is less overspray.

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1.2 is probably best. I wouldn’t go over 1.3.
Just to clarify, everything I’ve said applies to clearcoat.

I’ve never used the tekna so it’s hard to comment, I’ve heard it’s a good gun. I would think that the air cap, nozzle and needle would be matched so only 3 combinations? Maybe Joey can comment.
The real difference with rp vs hvlp is that the rp has a good enough transfer efficiency to be exempt from the rules requiring hvlp, it’s considered compliant so legal in areas requiring hvlp. So I understand, correct me if that’s incorrect.