Hi Bumble,
More awake time does mean more calories used up so slow or static weight gain as just because they are awake doesn't - as you have found - mean they will eat more.
Do you use a sling like a Moby or a Ring Sling where he would be held close to your body? this would be a good solution for the 4pm grumps especially if he was snug on your back as you can read to DD, get tea prepared and he can nod off for a nap.
I would hesitate puttng him to bed much before DD as this will restrict your family activities and mean he will continue the 5am waking, it is also nice for them to have a shared time with the 2 of you.
Given that your have DD's routine to work with also and the fact that with playgroup sessions it will take a little juggling
Do you have to BF him to sleep unless he is in the car/pram?
If yes then i would tackle that first to be honest as it is one of the reasons he is unable to settle or sleep for long, he links nursing with sleeping so the stopping of 1 means to him that the other should not happen - does that make sense?
I would have a plan of when he had BF and when he has solids the solids would be with dd, tho i would try to narroe the lunch window for him by setting a 15-30 minute window and having food like a fruit that can be cold with you so his intake is regular.
Has he recently become more mobile? that also can slow weight gain.
I can appreciate that a fixed routine isn't for you and you have dd to conside also.
I would suggest you print off one of these :
30 minute day plan with 3 columns(DS>DD>You)
or
30 minute week plan would need to use colours for DS>DD>You
and fill it in to give yourself the best idea of times it would eay to be home for DS sleeps and then you can also see about jiggling it for the odd days, so not a routine more a clear way of you getting everything down on paper with the whole family included.
If you want to download and fill one in and email it to me for help, feel free
matnanplus at live dot co dot uk