Delivering with one leg back is usually possible, depending on the size of the shoulders. Swales and Mules, and even Texels-out-of-Mules I've managed no bother, but the very chunky pure or near-pure Texels and Beltexes just might not come that way.
When you've got just a head you can usually push it back in, even when the head is very swollen, with lots of lube and patience. (And something to stop the ewe pushing back, if necessary.)
I don't think it's a good idea to push the lamb back in by the head if one front leg is back. You'd be pushing the backwards leg against the grain, I'd be worried about tearing the ewe. So if it won't come one-leg-only, you have to get your hand up alongside the lamb and pull the other foot forwards. The vet can give the ewe something to stop her contracting/pushing, which might be necessary.