We do not have the opportunity to go down to sister the joists

You would if the existing subfloor were removed.

or laying a half inch layer of plywood on top

What will that do too room to room transitions, doors etc?

after it is glued down and leveled as best as possible

After what is glued down? How will the floor be leveled?

In a similar situation we went over all the diagonal sheathing with 3/4” osb sturdifloor before installing 5/8” engineered hardwood. Floor-leveler was still required

You can always just patch in the holes that are in the subfloor. You don't need a entire new layer added.

x3 What @HALLETT & Co. and @G & S Floor Service suggested, I did a few older houses with board sub-floors some I used 1/2" plywood, and some were sub-floor was in a better condition right over the original, and never had issues.

Well sistering the joists is a must if there are issues with them , adding 1/2" plus the new flooring can have many repercussions as mentioned I am not sure what the purpose is in doing the 1/2" if those boards there are in decent shape , not pretty but solid . Patching and install new can be fine. I assume there is no basement so is the crawl space insulated I need abit more info . We ripped out the old in this house and installed new 3/4" becuase we were planning on slate floor,long story ,but we ended up with laminate but the sub floor sure is solid , but we checked all the joists in the basement that was open to make sure all were good .I guess I am a bit of a PITA about making sure all is right before the pretty stuff so I would probably rip it up fix the joists and then new sub flooor

1/2" over the joists is not nearly enough subfloor. No how, no way. Over the diagonal that exists? Maybe. Depends on how much deflection exists in the current patchworked assembly. And that depends on the joist spacing, height, and span. This is basic engineering and you should consult a deflection calculator to make sure that assembly rates at least a L360.

I agree. Remove some of the diagonal at least, so you can get under it to sister joists. You dont put extra weight on, unless those joists are solid. Its very important step, so dont let them talk you out of it. I would definitely want plywood on top of that flimsy diagonal floor, or remove all the diagonal, and put a nice thick plywood floor there. Kitchens have a lot of weight, with cabinets, and appliances etc. You need sturdy floors. It appears you may have removed walls. Were those walls load bearing? If so, did you have a beam installed.

That whole wall down with no beams, patched up subfloor, and who knows what for joists is a recipe for a home collapse. You need an engineer in there sooner than later. And an entirely different contractor. You also probably should not be living in the home until it can be verified to not be a death trap.

Your contractor is a hack. That is all done poorly, if not improperly. The no beam bit is especially worrisome.

Thanks everyone. You’re not seeing the entirety of the space. We did have an engineer and architect, we did seek all our structural permits, and we have ensured that we’ve got all that done properly and load balanced. Really appreciate the advice here and the caution surrounding adding half inch plywood and then our floors - both from a weight, room transition etc perspective. We will bring up this subfloor, do the work underneath and then lay 3/4 inch ply.

Proper patching isn't nearly as concerning as the diagonal placement of the floorboards. This increases the subfloor board span between floor joists which will contribute to floor flexing. Either solution your contractor recommends will work, especially when the new floor is engineered 3/4". Pay attention to the fastening schedule please. How will the addition of 1/2" affect the transition to adjacent rooms?

"That whole wall down with no beams, patched up subfloor, and who knows what for joists is a recipe for a home collapse. You need an engineer in there sooner than later. And an entirely different contractor. You also probably should not be living in the home until it can be verified to not be a death trap."

Macdonald:

There is absolutely no chance of a home collapse and absolutely no need for an engineer. Your hyperbole is not helpful.