I made some tests about the CO2 issue, no problems in my installation.
I have W10, I go to desktop via "window" key, no pause in the game, see some things in Chrome (SUBSIM.com, Facebook), then maximize SH5, ever submerged, then surfaced, CO2 recovered to normal. No suffocation in surface navigation.
I will test this thing about hydros.
Best regards,
Fitzcarraldo