To answer your questions:

-- Subscription is tied to account, not platform. You can buy the PS3 version of the game again to get access to it, or buy the PS4 version without transferring in the first place.

-- Everything you quoted is account-bound. The only thing that isn't is your action-bars, gearsets, and anything you modified in the Character Settings window (possibly more). These are tied to the console/PC you are using, not your account. Nothing progression-wise is lost when you move to another platform, as everything else is kept across them. I have to copy my settings between my laptop and desktop all the time, and this is also why I don't use my PS3 copy much (I don't want to have to set all this up again).

-- I think if you do the one-way transfer, you will want to buy another PS3 copy. The transfer effectively turns your PS3 copy into a PS4 copy. If you want to play both, buy the PS4 version without transferring (though you may have to set up your UI and gearsets again).

-- All versions that have and ever will exist will have these, as they are account-bound.

So basically, it seems your issue was thinking what you do is tied to the PS3 version. It is not. It is all cross-platform and you will lose nothing going to the PS4 version without transferring, or even the Windows version if you chose to, other than your UI settings and gearsets, as these are stored locally, and can easily (yet tediously) be re-set.