Most of what your getting as conflicted info is from people that dont have a clue, dont know, dont have a sump or have never run Co2 or a Sump together. Ive done it just fine Unsealed. Its no different then a wet dry really. Tom Barr Runs them with Co2 as well no issues.
Remember the bio-filtration and your living creatures in the tank all need O2, With the Sump set up your going to provide them wonderful O2, Yes you will have some off gas exchange BUT not enough to make a big deal difference. All you will have to do is compensate a VERY SMALL amount for what your off gassing. Its no biggie and remember Co2 is cheap.. I get my 5 and 10lb tanks filled for $8.60. My 20Lb stuff is about double that. You will get about 3 months out of a 5lb, 6 out of a 10 and about a year on a 20lb.
Really its not a big deal. I would highly recommend doing a reactor on the return line from the sump. I like the Cerges reactor. It will make it so you get 100% dissolved Co2 into the water. Easy to use. Works well.
Anyone though that says you cant run a sump with Co2 is in left field..