As a HX, mass-flow, psychometric weenie (Chemical Engineering / Berkeley), I have a number of thoughts.
1) absolutely, this will reduce condensation in your bivy,
2) it may take a while to adjust to sleeping with the mask on, but users of CPAP machines (my wife is an MD boarded in, among other fields, sleep medicine) have to deal with much more and they adjust.
3) you definitely took the right approach by using check valves close to one's face. You could share the tubing to reduce weight/bulk, but shared (in = out) tubing inceases the "tidal volume" and while reducing condensation in the bivy just as much, it would increase your uptake of CO2.
4) the reduced CO2 in the bivy may be the biggest benefit. I find I react (unfavorably) to the CO2 when I tuck my head inside my sleeping bag then I do to the humidity build-up.
5) the vinyl tubing you used is cheap and widely available. But HPDE tubing (as used in a radiant slap floor) is tougher and slightly lighter in the same diameter. Readily available in 100- to 300-foot rolls, Home Depot will sell it by the foot. There's 1/2", 5/8" and 3/4" available for residential appilcations and larger sizes for commercial use.
6) the length of tubing is a trade-off. Shorter is obviously lighter and offers less restricted flow. But longer gives you more flexibility in sleeping positions.
7) I strongly recommend you wrap some duct tape around your water bottle so you have some repair supplies handy. Even better is the 2" wide, red plastic tape used to seal vapor barriers in homes - it is stickier, stronger, and lasts for years instead of months.
8) **advanced concept** if you consider a tube-in-tube configuration for greater heat exchange or reduced number of tubes, here's the punchline: for X diameter in the inner tubing, you'll get equivalent pressure drop in a length of 2X diameter for the outer tube. e.g. if 5/8" ID works, you'll need 1-1/4" ID for the outer tubing with 5/8" inside of it. That seems really big with a lot more cross section but it is because of the increased "wetted area" of all those tubing sides exposed to the air flow. Probably better to tape two 5/8" diameters together an forego the HX. Maybe with a stick or lightweight plate taped to the end so it stays outside the bivy
8.1) There are HX masks with two check valves built in sold for aspiring avalanche victims so they can breath in and out of a chest-worn diffuser even when buried in snow. You've achieved something similar at a lower cost and weight, but you might look at those units for inspiration and/or parts. For high-altitude work, I like the how such a rig can capture heat from out-going breath and potentially recapture some moisture as well.