First, I don't believe that the internals and nozzle for that mini-jet assembly is interchangeable with anything else. So you may be stuck with that size. For a 5/16" nozzle, Waterway recommends 10 GPM for moderate strength and 12 GPM for a strong jet. So for 5 jets, that is 60 GPM at most.
#1 - Your current setup with the 1 HP NS pump should have provided 58 GPM @ 73' of head which would have been 11 GPM per jet. The strength of the jet should be moderate but working properly so yes, your current setup should work. Also, the suction for that would have been only 9.8" Hg., so it should not have opened the SVRS vent if that is really what you have. The bigger question right now is that your current setup should work but why isn’t it? I believe that air is entering in from somewhere that is causing the pump to lose prime which is the biggest problem. Fix that first, and you might find out that the jets are fine. Try plugging the air vent and see if things don't work better. Also, are you sure that it isn't the venturi air inlet? If not, do you know where the air inlet is; close to the spa or near the pad? If the air inlet is near the pad, then you need an air blower.
#2 - You would benefit with larger pipe but it may not be necessary to get things to work properly. You may find out you have a collapsed pipe somewhere which is restricting flow on the suction side so it needs to be replaced anyway.
#3 - There is no need to run the pump at full speed but if you don't, there may not be a need for very large pipe either. The whole reason to go with large pipe is reduce head loss at high flow rates. Running at 60 GPM or lower, really doesn't require that large of a pipe and under most situations 2" would be fine but at 95', 2.5" would have been a better choice.
#4 - I am not yet convinced that you have to replumb but again, it wouldn't hurt. As to the size that is up to you but see the analysis below.
BTW, did you verify that there is indeed a 5/16" nozzle inside of the jet? This is key to understand the expected performance of the spa. If you can confirm that, I can tell you within a reasonable amount of error, what kind of performance you can expect with each size pipe. I went ahead and assumed that it is a 5/16" orifice and ran some cases with various size plumbing. All of these cases assume the Jandy JEP 2.0 which I think you will be using no matter what. I also assumed that you would set the speed such you get 60 GPM no matter what setup you end up with.
Current plumbing (95'-2"/2"): 3360 RPM, 60 GPM @ 75.6' of head, 10.6" hg., 1678 watts (note that even this config should work)
Config A (85'-2.5"/2.5"): 2991 RPM, 60 GPM @ 58.4' of head, 3.7" hg., 1273 watts
Config B (85'-3"/2.5"): 2940 RPM, 60 GPM @ 56.2' of head, 1.7" hg., 1223 watts
Config C (85'-3"/3"): 2871 RPM, 60 GPM @ 53.2' of head, 1.7" hg., 1157 watts
Config D (85'-4"/3"): 2839 RPM, 60 GPM @ 51.9' of head, 0.5" hg., 1123 watts
The biggest benefit is going to 2.5" plumbing and each step above that is a modest change. Also, remember that each of these configurations has exactly the same spa performance. The biggest difference is how much energy they consume.
But again, it is really important to find out what the current problem is first. You might end up replacing all of that pipe and find out it is something to do with the spa jets themselves and have to tear apart the spa to fix it.