No no- i didn't mean tht 45 mins was wrong- that's a fine time at 3 weeks, you know your LO- some of them can only handle a feed and a nappy change.
I would TRY to be firm with DH and give him a task to do like the bath- and even encourage him to do the bed time, but keep the A time short! What you know about sleep means that you KNOW that she will be better rested and a happier baby to be around if the sleep is right. So don't obsess- but keep firm with DH- it's nt crazy mumma- you're the one who has to deal with it all day! Perhaps you could suggest to DH that DS needs some one on one daddy time and let him manage your DS's bed time and such- so after a short play just take the baby and send DH to manage DS... (not sure how this works time wise). Your DD really won't miss that much daddy time- but your DS probably will! It must be a hard juggle!
As far as waking- i would generally not let her sleep more than 3 hours through the day at a time as more than that she might confuse day and night. 3.5 hours is ok every now and then- but i'd watch out for more. Other than that- i'd just go with it!
Also- really try not to stress too much about sleep and independant sleep- what you hopefully know from your DS is that anything you do can be undone when you need it to be- so don't stress if you APOP a few naps, you'll have the tools to fix it later.
Oh- and don't worry about a consistent wake time- that will come in time, and don't count naps at this point- you are just going with a rolling easy- 45 mins up- up to 3 hours down, feed on waking and repeat! It's going to be a few more weeks till you have something that resembles the easy in the book- and that's totally fine and normal! Embrace your inner 'go with the flow' person!