A few pointers for Diafine: the speeds they publish on the box are often reasonable starting points but usually a tad optimistic. OTOH, while I know people who claim to shoot films at box speeds in Diafine, I doubt you'll be happy doing so with the films that get the most speed boost - Tri-X being one of the best with this developer. So it's going to take some experimentation, but that's easy enough. It definitely works better (in terms of getting more effective speed) with some films than others. Expect negatives to look a little flat compared to what you're used to. If you're scanning this will be a complete non-issue, if printing optically you may well need to print on a harder paper grade than you're used to but, again, not an issue. It's not going to need a grade 6 or anything, usually grade 3-3.5 works well, but I also print with a condenser enlarger. This can be a good thing with some films and definitely in some lighting.

Be very careful about contaminating any of the A with the B, as they say. The other way (carryover of A solution into B) happens in the course of regular use anyway and is of no consequence. Do not over agitate. I just invert and twist once per minute. You can always give a little longer than the specified 3 minutes as development just goes to completion. Most films are complete in 3 minutes per bath but not all, so if you want to develop different films together and one requires 4 or 5 (per box instructions) just give them all the extended time. You will not see any difference.

The box warns about this too but it bears repeated - do NOT rinse between the A and B.

It's really simple. I occasionally see people asking about "why use this more complicated developer..." or saying they like to keep things simple. Considering the virtual absence of effects from time and temperature variations as long as the minimums are followed this is actually the simplest developer that I've ever used, and I've been experimenting off and on since the late 70s.