Use a stop bath.
Pre mixed dev and fix last a fair amount of time.
Welllll, it depends on the type of developer you use. I am a bit old-fashioned, I really like Kodak D-76 (similar to Ilford ID-11) because it works with everything (I mostly shoot Ilford/Harman films). But it's a pain to mix. Once mixed I divide it up into quart and pint bottles. Once a bottle is opened and exposed to air, the clock is ticking, but bottles that are full up should last a while. And if it gets too old... a packet of mix costs US$10 (Legacy L-76, the generic version, is $7) so it's no big deal to throw it away.
That said HC-110 does have a shelf life of forever and it's easy to mix as you just add the liquid concentrate to distilled water. I don't like it for all films -- the developing time for HP5 IMHO is a little too short, and if I use a greater dilution to expand the time I think the negs are too flat. My plan was to switch from D-76 to HC-110 but now I always keep D-76 around. I'll switch back if I can ever use up the bottle of HC-110, which at the rate I'm going will take years!
The type of developer is personal choice.
Yes, exactly.
I don't shoot T-grain films much (I like grain!) so I don't know what the recommendations are for Delta. I do know that Kodak used to recommend T-Max developer with their T-grain (T-max) film but I've developed it in D-76 and it's just fine.
Not to confuse matters more but BTW people have been rave, rave, raving about Adox XT-3 which apparently their version of Kodak XTOL. I just bought a packet (of XT3; never tried XTOL) but haven't gotten around to using it. Rodinal is popular too, so I guess those are the Kodak biggies: D-76 (and the Ilford equiv., ID11), HC-110 (Ilfotec HC), Rodinal, XTol. Not sure if anyone considers T-Max one of the majors...
De-ionised/mineralised water will help prevent calcite patches.
I always use distilled water for everything except washing (I live in California where we get hard-ish water). And I use Kodak Photo-Flo for the final rinse to prevent spots. It's cheap and it
usually works. A lot of people recommend squeegeeing negatives; I don't but I do sometimes get water spots that I can later wipe off.
That's a new one on me. Mostly I develop the way I learned in the 90s -- developer, stop bath (Kodak Indicator), fixer (Ilford Rapid Fixer -- I like its quick low-water wash regimen, and we don't need hardening fixers anymore), Photo Flo, then dry.