Forgive me if I repeat others, but I pretty much only your posts. Sorry you & your dw are having such a hard time - sending big hugs.
My first ds was a big baby 97% & we too (3 years ago over Xmas) suffered a pretty stressful family situation at the 4.5mo mark & I was doing all the baby rearing on my own (occasional sitters) while visiting dh in Hospital daily, then doing the home recovery, so I can sort of understand how your wife feels about it all & how she is running on empty even before she starts. Although ds was doing 5-6 hours at night, so I at least got that break, but I though I'd share some ideas.
The other thing is your dd will be strongly picking up on the anxiety around her, so your dw needs to make sure she trys as hard as she can (not easy I know) to do a bit of relaxing breathing or something that can relax her a little before both bf & putting dd down for naps ...
What I did was "controlled breathing" where I took 3 big deep breaths then breathed in to the count of 3 & out to 3, then slowly aimed for count of 4 & then 5, both as I was feeding & when doing wind-down before naps. If your wife decides to do this I wouldn't be surprised if she has to start breathing to the count of 2 to begin with... incidently it has been one of the best skills I have gained, I just don't rememeber to use it as often as I could.
Another thing was I stuck to was Bedtime routine was consistent & I didn't allow him to fall asleep feeding, to drowsy yes, but he went to bed & I patted him off the rest of the way... if she falls asleep on the breast when she goes into light sleep phases that is what she expects, so if you can get your dh to try & get dd off the breast before she is asleep, but drowsy, that may help.
Also while she is big, rice cereal does fill them up more than milk but as it is proportionally lower in calories it doesn't necessarily stop the hunger & last as long as bf or ff & if she is only 5mo, it may also be upseting her tummy & making the waking worse... as you've started I wouln'd suggest stopping, but I'd suggest trying it earlier in the day (after the first feed so that it doesn't impact on bedtime & doing a bf with formula top up or formula with bf for the bedtime
Interestingly my second son who has always been off the charts height & was for weight until 12mo from 4-6mo could actually could go 11 hours without a feed & only fed 4 bf a day... what worked with him which I also did with ds#1 when he was 6mo (which kind of goes against BW) was to semi cluster feed in the evening... I would let him bf from one side before his bath (6pm) & then the other after bath at7pm
I'd think routine something like would be about right for a 5mo
7am bf + rice cereal
9.45 -am Nap
11am milk feed
12.30pm nap (earlier if more than 2 hours since last wakeup)
3pm milk + cereal
4.30- 5pm 30min cat nap (at 5mo most babies still need a catnap at this time)
6 pm Milk feed
A time with 6.45 top up feed
7pm Bed
That is a pretty textbook routine & it may need tweeking to fit other factors, but at 5mo the MAX I would consider A time should be 2 hours, but realistically closer to 1.75 would be better, as an over tired baby does wake more & harder to settle.
HTH
