OK, here's another thought that solves the problems I've placed in your plot. It isn't fair that I pointed out a flaw and don't help solve it.
Here we go...
The arrow shot by Isaac (thanks for giving me the bad guy's name) is at a speed that will kill a man if it hits, so just hit C's horse. Kill the horse, have it fall, and have her tossed onto the ground. Then, all you have to do is have her maybe dislocate a shoulder (which can be poppled back in by someone who knows what to do) or have rib fractures. Then, the story continues as you've written it. The arrow is taken from the horse to examine. The couple goes to Dacre tower. The smooch interrogation goes on schedule, and Isaac has the same time-constraints to make trouble for Matthew.
Rib fractures mostly heal within 10 days. After that, the pain is mostly gone except for sudden movements/ reaching and bending. (this might go along with your story later in the chapters). They take about 6 weeks to heal completely. A rotator cuff injury is a matter of putting in a sling and then slowly mobilizing it. C would have a hard time reaching the arm over her head until it was healed.
So I think the rib fractures is really the way to go and eliminates most of the stress that I've put you under.
FYI, I'm wondering if we should be sending private messages back and forth instead of using this forum. That way, I get notified by email that you've sent a message instead of potentially missing the new post that you put here. It also has a linear format where the messages get displayed in a line, so referencing the info is easy. Disadvantage is that we won't have others nosing around on the site and commenting so you won't have a social a thread if we move to private messaging.