It's certainly doable.

Consider whether to book assistance for DH or not, they usually give it
from the point where you hand in your own wheelchair/scooter, so I think
it would probably only be from the check-in desk onwards. The advantage
is he can take a bag or two on his lap if he does that - I can see much
of a disadvantage, other than being dependent upon others.

Here's how I would handle it, pack as light as possible and have at
least one carry on as a backpack your DH can carry, only take stroller
and/or car seat if absolutely necessary, consider hiring a car seat at
the other end if that is the reason for taking it. Arrange baggage such
that once checked in you can manage everything but the backpack, this
might mean making compromises to usual standards, such as not using a
car seat on the airplane, though you could probably still manage that if
you had the car seat strapped on to the stroller, then DD holds the
stroller or reins whilst you push. For the section whilst you have all
luggage, use a trolley and put everything on it, again DD holds on and
walks along side, or is on reins.

Kids learn fast when they need to, you've been lucky so far to have the
luxury of on adult for the child, one for all the bits, we've travelled
across the world with me in a wheelchair, 2 small children and
ridiculous amounts of luggage and still managed to get 1 car seat on to
the plane (kids were 1.25 and 3.25, both had seats and the younger had
her car seat on the plane).

Another idea would be to have DH look after DD, it is possible, even on
crutches, you can hold reins and still hold crutches, or just control
with your voice, whichever works best for your child. It's not an ideal
situation and you don't have the back up level of safety you have in the
ideal situation, but it can work, DS very quickly got the idea of
sticking by me whilst I walked on crutches and we were pretty confident
in a wide range of situations.

Anne