With a pH of 7.2 I'd get that up too by adding borax. It's a little too close to neutral to be comfortable for the winter. Liners don't like acidic water. With no pump I'd dissolve the borax in a 5 gallon pail, maybe 1 pound at a time, warm water would help, then pour it around the pool and disperse it like any other liquid. If you go the sump pump route I'd advise sucking from the steps at the shallow end and discharging at the bottom of the deep end, as far from the pump as possible. My educated guess says the reverse would be ideal, suck deep, discharge shallow, because of the higher density of the borax mix. But, it's usually easier to have the pump shallow and it probably makes no significant difference. The pumps are generally very low power so you could let it run a long time without running up $$$ in electric costs.
Al