How can I run air tubing into a PVC manifold / fitting?
Assuming you have or can get the clear tubes to attach and not leak, I'd suggest a 1-1/4" (or reducer and 1/2") PVC pipe parallel to your clear pipes, running above water level, and NO penetration for the CO2 feed line; not only run it in the bottom of the pipe as suggested by @bib, but run it inside regular, normally solvent-welded PVC vertically as well, all the way above waterline. No funky or fragile seals between plastics that don't solvent-weld nicely.

From fishtank experience I'd also suggest that you be open to running 10 tubes so that you can balance the airflow if it does not opt to cooperate and divide evenly in practice (one of those things where things are the same in theory, but often different in practice).