I'll try my best to answer with my experiences so far, and I'm also going to make the assumption that you are wearing the pf cap that came with the device only, not any extra sleeves on the shaft, correct me if I am wrong there.
Fluid retention - where is the fluid buildup? I would guess it is behind the coronal ridge or near/around the frenulum? This is going to happen in any device that has a vacuum, lowering the pressure inside the bell or easing off on the belt tension should minimise that, as Mike mentioned above, putting the cap on while erect should stop anything bunching up under it when putting it on, So long as your dick doesn't look like it's wearing a flotation ring on it after your session any minor fluid should receed within an hour or so.
Swelling urethra (vertical bit between the eye and the frenulum?) - I've still not figured this one out, the only way I've countered it is to press down on it and deflate The swelling with my finger after I take the device off in between sets of wear time, shit is uncomfortable though!"?
Blisters - the only time I got a blister was wearing the silicap, I had an air pocket in it that I ignored and sure enough, blister ? the silicap2 with the vented tip would 100% eliminate that possibility.
Slippage - you are losing pressure somewhere along the way, it's going to happen to some degree when you are moving around. I am usually just bumming around the house while wearing mine, more often than not sitting in front of the Xbox so I'm pretty still most of the time. Try lowering your belt tension slightly and see if that helps, if it is still slipping then you need to look at getting a stronger seal with the condom against the shaft, hence the main reason why I wear a silisleeve, it provides a smooth surface that is wrinkle free when rolling the condom on, therefor eliminating any extra air trapped and once the seal is achieved, it stays that way til I take it off.
Ideal tension for long sessions? I Stick in the green about 3/4 along, bordering on yellow, why?, because through trial and error this is where I can still feel the stretch happening without my glans being super sensitive at the end of the day after many hours being exposed to the vacuum. We are all different in our tolerance and resilience to external forces being applied to our body, pick a spot, try it out and see what happens, you will know if it's too much or not by the length of time you can wear it comfortably.
When you say allowance I'm not sure whether you mean at the tip or the base of the cap? I'd give a couple mm clearance from the tip to the top of the bell so you don't damage the cap sucking it into the holes in the tip of the bell, if you meant the base, it doesn't matter, as long as all your glans are inside the bell you're good.
Sorry for the lengthy reply, I'm not great at short answers ?
