I'd say there is a significant chance one or more of your ewe lambs could be pregnant.
Hoggs and a single lamb isn't such a bad thing - but I'd keep 'em handy when lambing as they sometimes need to be penned up and given some support to get the lamb to suckle (and be allowed to suckle) and to bond.
We often tup a few hoggs, the bigger ones, and later in the season - they learn their jobs and make better mothers as shearlings, but if they have twins it's best really to take one off and bottle-rear it, two takes too much out of a hogg.
It sounds as though your prospective buyers are novices themselves although they have great support to hand. I really wouldn't suggest in-lamb hoggs for novices; you need a bit of experience of lambing to cope with hoggs lambing, I'd say.
I could suggest a way to find out (pre-scanning) - if you could get hold of a teaser, raddle him, and run him with the girls, anyone whose bum he paints is not in lamb and could be sold. Anyone not painted is either in lamb or not cycling yet.