What grade sandpaper would give me the best mechanical bond and not risk going completely through the seal coat??

The seal coat can be sanded with 100 grit[go with the grain] .There is no worries about sanding too much by mistake because the reason for the seal coat is to seal the micro pores in the wood so the air in them doesn't become an issue when doing the next coat application if it is being applied along with raising temperatures.You wont be able to sand out the seal coat inside the pores.Plus you will be getting rid of excess weight.Good things usually take time.
cheers,