My DH is away all week often (though is around a lot when he is around) and LOADS of people we know have DH with working patterns at least as tough as yours.
I had a (difficult) just 2 yr old when I had my second, and though it was at times frazzled, I did fine with no additional help during mat leave. HOWEVER I really would not have wanted to manage the 8am nursery run with two of them. TBH, on the one hand I think you're worrying too much (esp about the tea-bed time slot), OTOH, any extra help helps, and I don't see why you shouldn't get it in if you want and can. Our solution was to get a nanny, which really makes life lovely. Have you considered doing that, leaving your DS in sessions at his nursery rather than full daycare?
On the mother's help side, I think that a local teenager / student is a good bet, or a youngish retiree. Ask round and advertise at the college - try calling your local college doing childcare courses and ask if they can suggest someone - they would direct you to someone reliable not a waster! I'd expect to pay similar to cleaner rates, though you'd need to consider whether you really should employ properly with payslips / tax etc in which case you might need to offer a higher gross.
Good luck