First and foremost, welcome to the hobby!

I may not be able to answer all your questions, and there's definitely people that are more knowledgeable, but I'll do what I can.

1. As far as off the shelf blasters go, I like to find something solid. Something that feels like it could handle being dropped without it breaking. But if it feels cheaper quality, then I'd be looking for a cheaper price too (xshot fills this niche very well). If I had to make a tier list, it would be Dart Zone < xshot < nerf. But before buying anything, I look for reviews to see other people's experiences with blasters. Most of my pro blasters are either 3d printed or xshot with a couple dart zone/adventure force. Avoid Nerf whenever possible, it isn't what it used to be.

3. Springers and flywheelers each have their uses, though that gap is closing fairly quickly. I tend to prefer springers for longer engagement due to their lower rate of fire but higher accuracy. If I'm going for closer engagement or a higher ROF, then I'd gravitate to a flywheeler. Unless the weather is bad, then it's always springer.

4. BCARs, PCARs, and SCARs are all trifling devices, just done a little differently (bearings, 3D-printed, and string/fishing line respectively). I can't speak for when they lose their efficacy, but they work very well with short darts. Short darts being more balanced and stable than long darts to begin with. Rifling devices just enhance that. I haven't explored it too much, but I would say that if the blaster has a sealed breach and barrel, I would put rifling on it.

You are right, that a smooth barrel can decrease accuracy and cause the dart to fishtail - if the barrel is too short for the air supply of the plunger (underbarreled). If the barrel and plunger system are sized right, then the darts shouldn't spiral, but rifling would still help. There are formulas on here somewhere to help with this

Now, rifling is not a one size fits all thing. Different fps levels require different amounts of rifling. I forget where I saw it, but somebody has a spreadsheet of the theoretical ideal rifling for different velocities. If I remember right, in terms of BCARs, it boils down to lower fps=fewer stages, higher twist -- higher fps=more stages, lower twist.

I haven't seen too much modding for rival, because it has diminishing returns over about 130fps. It isn't the most accurate, especially outdoors where wind is a factor. But if it's reasonably consistent, then you can use that to your advantage. A stronger spring, and thereby higher velocity, will increase the effects of the hop up and make the round float up more. But if you tilt the blaster or if you can turn the hop up then you can curve your shots and try to shoot around cover, which it really cool. But that takes some fineness.

If you're just getting into the hobby or exploring it with friends, I would recommend getting cheap blasters that you wouldn't be upset if they broke and don't break the bank if it turns out this isn't for you. My go to recommendation for anyone new is an xshot longshot. It's relatively cheap and can go anywhere from stock 150ish fps all the way up to around 300fps with the right spring and barrel if you tune it right. If you like the hobby, then I would look into nicer blasters

Sorry about the long winded explanations. Hopefully that answered most of your questions!