I would personally opt for the heartwood, just for the aesthetics. I don't think there is all that much difference between the properties of the two.
Moisture wicks into any stave that is sinew-backed, you simply need to give it sufficient time to dry back out. That means weeks, not days. Not to say you can't pull the bow after a couple of weeks of drying, but it will continue to dry over the course of several weeks. If the ERC has been kilned hard it might be brash, but today's kilning technolgy generally produces satisfactory bow wood.
Sure you can sinew back ERC. It should behave something like juniper which was a preferred wood for sinew-backed bows on the West coast. It should be a really nice bow in fact. People have made selfbows out of ERC, so a sinew-backed bow is certainly a possibility. Good eye for finding the wood, and good luck on the bow. Be sure and use a deisgn that is specific for sinew-backing, that is, shorter than an unbacked bow could survive. Thats where sinewed bows excel.
