I'm not an expert and have only rebuilt a couple of engines myself. My suggestion was based on the fact that you are unlikely to be able to remove the rust pits full stop but I think the engine will still run fine even with them present. My concern about the emery paper was that it would destroy the honing pattern. A sweep of the piston would remove any rust on the surface of the cylinder wall whilst not affecting the honing.I've rebuilt a few engines so aware of the cost which is why relining is preferred route as a I can keep pistons and just fit new rings.
Apologies Teflon if I sound frustrated, I'm not, I just don't understand how barring the engine over will get rid of those rust pits. If a gentle rub with some emery paper exposed them, then IMO scraping them with piston ring would also do the same.
Either way it would leave pits and comprise new rings.
I've now removed everything from the block and will try lowering it and sliding it under the wheel arch the next week. There's a machine shop nearby in Daventry that might be able to re-line. If worse comes to worse, there's a good used block on eBay for £230