The kyphosis doesn't surprise me. Be sure to work on thoracic spine mobility. You probably already know this - but avoid foam rolling the lumbar spine.
You don't have to overwhelm her. Just spend 15-30 minutes per day on one or two letters. Work at the pace she's comfortable with. Eventually, she'll be able to perform YTWLs effectively. You're essentially teaching her a new language, so treat the process accordingly.
Diastisis?!?! That adds a whole new level of complexity.
Getting her diet in check is so obvious I didn't mention it yesterday. But now I'm compelled to kinda beat you over the head with it.
You're right in that the way you or I would brace for a big lift can put additional strain on her linea alba - which can definitely make things worse.
I agree that strengthening the transverse abs with vaccums should be explored. Perhaps start by having her lie down on her back, with a neutral spine.
In addition, she'll need to learn mmc, proprioception, proper breathing patterns, rib cage to pelvis alignment, and even seemingly trivial things like how to get out of bed the correct way.
When she improves, consider progressing to sitting or standing variations on the vaccum. But it's CRITICAL she acquires the skills listed above.
As you stated, bird dogs are out, which I think are over rated to begin with. Now add her knee issues as well as the diastisis and it makes it an easy decision to drop.
And bb rows you mentioned in your first post, or even db rows, for that matter, are out. When she's ready, seated rows where she can place her torso against support is a better option. Watch her closely to make sure she doesn't hyperextend at the lumbar spine to 'pull' the weight back. If she does lat pulldowns, watch her closely and make sure she doesn't do a mini-crunch to finish the rep. Both these subtle shifts - which are very common - can put further stress on her condition.
Even the push up - in its easiest variation - should be avoided until things dramatically improve. Remember - when you're a doing a correct push up, you're simultaneously doing a variation on the front plank - which is obviously out for now (and don't even dream about crunches in any shape or form).
Db floor press is a safer option for now. You can spot her from above and give cues regarding packing shoulders, etc. Have her do it the standard way, then add variety by doing things such as two up, one arm down, same arm up, the other arm down, etc. Several variations you can work with.
Once she learns proper glute/quad/posterior chain activation, step ups should be something you can implement. But like the facepull example I gave yesterday, she needs to SAFELY learn how to engage the muscles involved (using clamshells, TKEs, etc.) BEFORE she integrates them into a complex movement. As she advances, further unilateral work will play a key role.
Anti-rotation work, such as Pallof Presses, might be feasible down the road. But hold off for now. It's a movement where you have know how to engage and keep tension from bottom to top.
Someone mentioned complexes. I'd hold off on that for now. As you stated, you don't want to overwhelm her. And, going back to my learning-new-language analogy, she's still at the stage when she's learning basic words. She's not ready to string together sentences. Down the road, sure, it's an excellent proven method.
This is what I meant when you have start at the most basic level and construct a scalable program.
Going back to her diastisis, you can look up things such as the Tupler technique.
However, I'm going to stand firm by my earlier comment. By hook or crook, find a qualified specialist with a proven track record. At least discuss this possibility with your better half. She's your wife but it's her body.
And if you two want to proceed in that direction, ask for references (watch for shills), check backgrounds, and narrow down the possible candidates. If possible, speak to them on the phone or via email to get an overall feel. If they have a website or blog, a close reading can reveal plenty.
Learn as much about diastisis and treatment options so, on your initial visit, you can ask intelligent questions that: 1) will further your knowledge on the subject; 2) sends a subtle message that you are not to be fucked with. As long as you're polite and tactful, no legit professional will get her panties in a knot.
And if that little voice tells you the person is a joke, thank them for their time and move on. Should you find someone you and your wife feel comfortable working with, be an active (but not meddling) participant in her treatment.
This is the course I'd take if it was my GF. But I realize your circumstances may differ.
Regardless the decision you two make, remember what I said the other day. When in doubt, go with the easier variation. Get constant feedback on any given exercise. Never go to failure or grind out ugly reps (c'mon, she's an out of shape beginner with two significant issues). When introducing any new exercise or protocol, keep the RPE low, wait a few days, and see how her body responds before proceeding.