Sounds like a teardown.

Simple based on what you wrote. Make sure there is fire blocking between the joists on the mid span steel beam. Remove bridging and install solid blocking at mid span of floor joists. Remove the underlayment (?) and re nail the subfloor. Depending on the "holes", you might have to sister a few joists first. Ignore decorators posing as "pros".

Pics?

What exactly are your concerns and what do you hope to accomplish?

The span for the 2x8s seem to be adequate, which is typically not the case in old houses.

The tilting can be a problem - difficult to straighten, but also that would mean you have low spots upstairs. Is the "tilt" what is causing the non-uniform spacing when you measure across the bottom of joists? By the 1940's you should have new nominal lumber (possibly SYP) which should make it easier for you to fix.

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Because the 2x8 are adequate, no sistering needed beyond shoring up swiss cheese joists. Besides, unless you are removing all MEP, you won't be able to sister.

Blocking is required for lateral restraint to prevent the joist from rotating, not fireblocking. This occurs at the bearing ends of a joist, so the rim joist takes care of it on the outside wall, and down the middle of the house you need 2x8 in between each joist over the beam. Joist bridging somewhere at midspan is not really needed here, as the rotational moment is not as great in the smaller section of a 2x8 joist (rather that a taller joist like a 2x12) and the shorter 12'-3" span. Adding blocking over the beam is adequate.

The tilting needs more info and pics whether that can have a fix.

The squeaking can be alleviated by screwing the T&G subfloor into the joists. Fixing the tilts may help this too. Any future flooring underlayments would not be fastened into the joists.

This can certainly be DIY for some people, but those 3 letters are a swear word for others.

Forgot to upload

I guess what i really need to know, not knowing whether the builders even used 16oc or just eyeballed it (no original bluebrints so no way of knowing. Should i just block them all as they are and just snug it up, or should I start with a joist that actually IS 16 oc, cut the normal 15 1/4 block and work my way down from there?

The joist spacing is not off enough to be a problem. Leave as is.

Start with the blocks on top of the beam. It doesn't look like the joists are secured to the steel beam so measure the distance at the top of the joists to get your block length, then maybe you can move the bottom of the joist so it's square with the subfloor. You can't move the position of the joists unless you pull all the nails in the subfloor, so not worth trying.

So i took some more measurments, the ceiling in my kitchen is exposed as I'm redoing that as well. So every single joist up in the kitchen is actually 16 3/4" oc.

A majority of the joists in the basement are between 16 1/2" and 17" oc give or take about 1/2" or less

So knowing that, i feel confident removing the crappy braces and blocking it with 14 1/2" blocks.

You should measure the distance at the top of the joist. Even if the joists were laid out correctly they could be off a bit.