Which trade(s) should I hire for structural repairs

Frankly, in an older home like yours seems to be, I would not try to correct so small a sag. But at the same time, so small a sag should be easy to fix.

Yes, the typical solution is just what you propose -- place one or two 6' jacks in the basement below the sag and span them with a 4x4, underlying 2 or more joists. And yes, cracked plaster is a risk. But if the current sag hasn't yet created a crack, it's likely that correcting your sag isn't going to move the wall far enough to cause cracking. Worst case, a plaster crack is actually pretty easy to repair; it's just a bit messy (patching, sanding, and repainting a wall).

In my opinion, correcting such a small sag is unlikely to introduce other problems (add stress to plumbing, electrical, shift door jambs, introduce trim gaps, etc). Consulting a structural engineer is definitely overkill. Most home renovation contractors should have experience with this repair.

"Getting to the Bottom of Sagging Floors"

"Fixing a Saggy Floor"

None of the three options you've shown are likely to add substantial strength or stiffness to your floor. To add either, you'll need a single, continuous additional member to span the majority of the joist length.

The blocking is probably not critical, especially if you're doubling the joists for most of their length. It's typically installed to prevent deflection, and that's not a significant concern where you have doubled joists.

My suggestion would be to remove blocking from every second joist space and double for 80% of the joist's length in those spaces. The remaining blocking will serve to stabilize all joists.

If the original joists are sound (not showing rot), compression is probably not a concern. Even a tub of water doesn't add up to enough weight to compress healthy lumber.

Regarding your comments on the original ideas, you will see some gains if 1) you fit your additional blocking extremely well, and 2) you fasten it all together with screws and construction adhesive. The result will essentially be a load-bridging honeycomb that will improve rigidity and strength. However, the flooring already provides substantial load-sharing, so gains will be minimal.

The primary reason that it's not widely hailed is that it's a very putzy, time-intensive plan that's ultimately inferior to a proper joist sistering. You'll spend more time cutting and fitting all those blocks than you would tearing out half the blocking and slapping new lumber alongside the joists.