Hi youngatheart, here are my thoughts. I'm pretty risk averse, and I've been a hiring manager for a long time, so that's going to influence my response.
First, congratulations on your education progress and your plans and options for your path forward. That is great, and it sounds like you're moving in a direction that will be more fulfilling for you than being a "manager" who is coordinating the efforts of expensive, in-demand DVM's. Working full time, school full time, and young kids is a tough challenge. Be sure to nurture your relationship with your wife through this.
With your wife's health uncertainty and your education plans, does it really make sense to continue making double payments on your mortgage? Having more cash/liquidity might be really valuable in the next couple of years, especially if you decide to cut back now, or if you defer your reduction to the future.
Giving up $13,000 per year in salary and another $13,000 in bonus potential is a pretty big loss. However, if it gets you and your family through the next 2-3 years of schooling and into a higher paying, more satisfying career path, it could all be worth it.
If your educational path is uncertain and there is a good chance you might stay in your current profession, stepping back could be quite detrimental. I know fellow managers and executives who look very negatively on anything other than a steady stair-step upward in career progression. (That has worked very well for them, so it must be the right {read 'only'} way that careers should progress.) If you finish your education, then it's an easy story: "I was working full time, going to school full time, and we had young kids. I cut back at work so that I was able to be a good partner and finish my schooling, and now I'm a PA (or have a BA) and am ready for a new work challenge!" If I'm interviewing someone, that totally makes sense and shows me you have vision, a conscience, commitment, and are hard working. If on the other hand, you don't finish your schooling, you will have given up the manager role, and it may be hard to get a second chance at it.
I wish you the best as you work through this. If you can make the finances work, it sounds like the change in role could be great for your family. Giving up ten hours of commuting will save money, and it will give you more time to dedicate at home and to finishing school. Keep your eyes on the long term prize, invest in your family and yourself, and you'll find the right path for you.