I lived in Kuwait for awhile, and during about 9-months of the year...the 'cold' water was too hot to use without tempering. We used the WH, which was turned off, as our cold water supply, and it was a major problem when the water in it was replaced with the hot from the roof until it had a chance to cool off again. We only used the WH for hot a couple of months a year. I think your solution is more bother and would suggest you just keep the line the same size and forget about trying to use it for heating. If I did my math right, 100' of 2" pex would hold about 16g verses about 2g in 3/4" pex. Personally, I'd prefer cool water at least out of the cold faucet, not hot.