You poor thing, you sound so stressed out.
Dont worry about the formula making Ivy constipated, 1 feed of formula in a 24hr period is not going to make her constipated, it will help your sanity though, your DH sounds lovely supporting you like that, he has the right idea, mix feeding will help you get through this patch, its important you get your rest, not only for your milk supply but for your infection to heal properly. Remember to take care of yourself, and let your DH look after you aswell X
I'm mix feeding Myles and its great for us, I express twice in a 24 hour period and get between 200-250ml each time so enough for about 4 feeds of EBM, the rest he gets formula, we haven't had a problem with constipation, or anything else. But if we do we'll change formula, there are so many out there so just need to find what suits him, we started on Karicare with no issues, but are trying S26, so far so good with that aswell.
Take care of yourself, and think of the formula feeding as binding time for bub & Dad XX