1. If you do go for the chain device then avoid the MRP 2x, it's a complete bag of sh1te and made of sh1te stuff that can't retain a chain. It's also made and distributed by sh1sters who quite clearly have done sh1te all research. Go for the new E*Thirteen:
Skid Bash Version
Full Bash Version
There are 2 different versions one of which can be used with a full bash ring but I've got the skid version and it does a great job. Depending on how your front mech is set up the top fin will also help stop chain suck off the front.
2. I tried sticking with a 2x10 front mech which was for a 42T ring when I dropped down to a 39T and even with the chain device I found the chain would jump off the top ring to the bottom. The smaller front mech offers some retention and I haven't had any issues yet