1/ Lifespan will probably remain unchanged. DL burning is not exactly physically taxing (unlike 8x burning). Though I can't be 100% sure of this, of course.
2/ With the exception of the 411S@811S, error rates don't seem to be higher when using the same features as before. Error rates using the "new" features may be higher. In other words, if you bump a 451S to a 851S, your 4x burns will likely remain unaffected, though that may not be the case for your 8x burns. For the 812S@832S, there should be no adverse effect on single-layer functionality, and testing seems to confirm this. The quality of DL burned on a 812S@832S might be in question. We won't know until we get DL media to play with.
3/ Higher heat? No. See point 1. The 812S@832S should not generate higher heat.
4/ Compatibility? See point 2. For single-layer, there should be no adverse effect.