I highly doubt you would be able to weld this part successfully. If I were in your shoes I would be investigating whether reinforcement with epoxy or a similar product would provide enough improvement in properties to do the job. The next step after that for me would be brazing the damaged area to fill the crack.

Neither of these methods are going to be great at preventing crack propagation.

I think the main problem is that plate it bolts to and the screws are carbon steel and the holes that are cracking the worst are at the edges.

As far as filling it with an epoxy, it needs to be porous to pull a vacuum all around the plate.

I talked to the CWI and welder and they laughed at me, haha

If you need full porosity through the whole part, than your only option is going to be to buy a new part. Even if you could weld it back together, you would be creating a zone with zero permeability.

You'll never get a clean weld - the vacuum chuck has probably spent many hundreds of hours sucking in stray grease and dirt from the machining process, fully coating all the internal pores of the material. The welder would fight spatter and lack of fusion forever when trying to lay down a weld bead.