For the transfer, using a plastic siphon is totally fine. The important part is using CO2 to move the beer and minimizing any oxygen ingress in general.
Dry hopping for me is typically 36-48hrs post-pitch, I haven't been "double dry hopping" very often, but it's totally reasonable to add an additional dry-hop in the purged keg you ultimately transfer. With that, you may not want to do it unless you've either cut your dip tube a bit shorter or are using a floating dip tube like the clear beer draught system (i'm a big fan of this option). Some people have reported some vegetal taste from keg hopping, I personally haven't gotten that -- but something to be aware of. Might be something to test after you get through batch 1 so you minimize the points at which something could go wrong!
Dry hopping for me is typically 36-48hrs post-pitch, I haven't been "double dry hopping" very often, but it's totally reasonable to add an additional dry-hop in the purged keg you ultimately transfer. With that, you may not want to do it unless you've either cut your dip tube a bit shorter or are using a floating dip tube like the clear beer draught system (i'm a big fan of this option). Some people have reported some vegetal taste from keg hopping, I personally haven't gotten that -- but something to be aware of. Might be something to test after you get through batch 1 so you minimize the points at which something could go wrong!