1. You get mil leave for either active duty or reserve. That includes unlimited leave for initial training AND any service obligation incurred in training. This means you could join the navy, do training, stay in for ten years or so, and then return to your airline with full accumulated seniority...as though you never left. Actually you also get five years discretionary mil leave in addition so you could stay until about the 15 year point. So you're not risking your regional job (unless they go bankrupt or furlough someone senior to you).
2. You would be giving up the major airline opportunity for a long time...given current industry dynamics, that's a lot of seniority and pay at the back end of your airline career. As much as I like the navy, I probably wouldn't do it knowing what I know now...golden opportunities are rare.
Instead, join the guard or USAF reserves...that way you're only out a year or two for training and then can resume your civilian career. Career timing might be a wash anyway, since you'll be able to check the military box and *should* get picked up by a major that much faster.