Good advice re the days out. Totally agree.. He does the days out on his own,. get him used to it and have a break at the weekend yourself!
Im not clear if he's back or asking to come back.
If he's already back use him as people have said. Although that really depends on how reliable he actually is in terms of childcare at least. If he comes back and won't do nights iwth the baby - what use is he?
In practical terms you have three weeks before return to work. Hang onto that job! I remember so well how hard it was to leave two children and return to work - but I was not working in a friendly or supportive place - but even then it is nice to have a change and working from doing childcare. However, although the new routine will be hard at first, things will very soon settle down. Don't let the return to work distract you from you main goal. Get as much help and support as you can during this time. If he's not there could a grandparent come and stay over for a night a week to help out? Or just come for an evening so you can re group and reorganise.
I was quite disorganised at first... but getting into a routine of having all the snacks/bottles sorted at the weekend or the night before... all the clothing and baby/todder bags packed the night before made all the difference.
Have a work tote, where you can bung all the stuff you might need as you go through the day so you are not looking as you are running out the door, so that you don't have to look for everything. Keep a spare set of work makeup/hairbrush etc. in there too. Your own clothes laid out top to toe so you just have to step into them. All these small things will help you feel more in control..
If he's already back and you think he might be of any use at all over this transition period then its up to you. If he's not pulling his weight.. its not worth having him there at all. Out he goes. You don't want a third child lying around doing nothing. Don't do any chores for him either.
In your first post you said "He’s been becoming more and more difficult and even nasty during this time which has made me desperately sad and confused."
His remarks making you feel unattractive and not good enough were despicable and he cannot be allowed to continue making them. (in fact him saying that was almost him putting the blame on you for his having an affair, because you weren't doing enough to be attractive to him. So those comments are worthless because well, he wanted to make you feel bad and as if everything was your fault.
Don't let him try any of this crap during this time. Or when co-parenting.
I'd almost treat him as you would a very very difficult colleague that you have to handle in order to get them to complete a task, don't let them get too familiar, keep at arm's length, don't initiate conversations or try too hard to be civil, or ask advice or give him any more excuses to put you down. He is too pathetic for his words to have any power over you any more.
PS.. DONT MAKE THE MISTAKE OF PAYING FOR THE ALL OF THE CHILDCARE FROM YOUR WAGES. If he's in the house, It's a joint expense - he's not allowed to wiggle out of that one.
Pay all your wages into your own account and savings. He puts children money in the joint.