1. Theoretically, if you can one look (then again if you can one look why don't you learn EG) or have extremely fast reactions and tps you can be sub-4 or even sub-3.5. I used to use LBL (I sometimes still use it now when I don't know the CLL case) and I averaged around 3.5-4 seconds with it.
2. Learning CLL then Ortega would be kinda dumb imo. Ortega is suitable for sub-5, CLL is more of sub-3.5~sub-3. I personally hate Ortega because in some cases you would have to do z2 because the top is solved and the bottom isn't.
3. If you want to be world class, then yeah, you should learn some 1LLS. I'm planning to learn some of that when I learn full EG. Plus, even though 1LLS has more algs than ZBLL, the movecount is much shorter, and some of them are just TCLL and CLL. If you don't want to memorize all of it, then you should learn some subsets, like memorize the algs in where a corner is facing up, facing right, facing front, etc.