Yes, it's probably safe, but it's not good food handling. If, for some reason, you want to braise in the same liquid you used as a marinade (completely unnecessary), you should strain the marinade, boil it separately for a few minutes at a hard boil, reduce to a simmer for a few more minutes, then re-introduce it to the food.
Neither, for several reasons, is your method good braising technique. For instance, there's simply no need to marinate meat before braising. Braising is enough. In fact, that's kind of the point. Also, the meat and aromatics should be browned before liquid is introduced.
On top of that, your temps and times are pretty much guaranteed to turn the ribs into mush.
Finally, if a liquid is loose enough to use as a marinade, it's usually not tight enough to make a good glaze, unless it's applied very thin.
Here's a better way to go about it:
- Make the marinade, reserve some if desired.
- Trim the slab(s). Remove the membrane from the back of the ribs. Place the ribs in the marinade.
- Drain the ribs, discarding the marinade. Season with dry seasoning (if necessary) and place the trimmed slab in a large roasting pan back side up. Cover with foil.
- Cook covered at 300F for 90min.
- Meanwhile reduce the marinade (if you'll be using as a mop and sauce) until it reaches a sauce consistency.
- Remove the cover, baste the ribs on both sides with the reduced marinade or with some other sauce.
- Return the ribs to the oven, uncovered, and front side up. Continue basting every 15 minutes.
- Remove the ribs when they are tender, probably another 30 to 45min after uncovering. Rest, portion and serve with the basting sauce.