the spinnaker is certainly legal in europe where it is usually raced as either a seperate class or on handicap with standard boats, i suppose depending on the numbers of boats.

as for snaking downwind the reason is, as you accelerate with the spi up the apparent wind shifts forward dramatically so you have to bear away to keep the spi filled, then if you slow down for whatever reason you luff up again to keep the power on. your not gybing as you do this so no need to change sides. you also have to do this in gusty conditions where you bear away to depower in the gusts

hope this helps
andy