What you're after is a holding tank and then you use a booster pump on that. Works in exactly the same principle as a power shower: Get as big a storage tank as you can in the loft, add a pump, then alter pipework to suit. The trouble you're going to have is that you need to keep it potable, which is invariably a tricky thing to do at the best of times. Tbh it's not something I've ever done at a domestic level, only at a commercial one, so what would be a reasonable solution for you I don't know.
As for part 2, see above. Exactly the same principle, except that this time you've got a cylinder between the pump and the outlet.