its fine to re-use spokes, as soon as you de-build a wheel, the spokes go back to their origional length. If they stayed stretched then your spokes would need constantly tightening from when you built the first wheel. as regards to the splitting rim, are you sure thats just not down to the spokes beeing loose at the join? Its not uncommon for there to be a gap in new rim, but when you build the wheel, it pulls the gap together. Grinding it won't weaken it either, it only takes off a tiny amount off the rim, it more kinda moves it around rather than removes it altogether.