What's the most efficient way to remove these undercuts for injection molding teeth in Solidworks?

Remodeling is likely going to be the best approach is this is going to be injection molded. Too many "repairs" is just going to leave torn up model surfaces which the tooling shop will dislike. There is already a weird defect on the rearmost tooth which also affects the gums.

If the changes are not too involved, it might be worth a try of the "deformation tools" in SW. I never use them much, but it they could push and pull the offending geometry into the correct shape it would be the fastest way.

How many of these need to be created? I know you are looking at injection molding, but for a smaller quantity there are other molding operations using soft (silicone) tooling which can handle undercuts. There are other limitations with materials though, so it can only help in some cases.

In addition to undercuts, you'll want to check for interference. Three of the teeth are embedded in each other.
There are also small "tunnels" formed by the teeth and gums which can't be molded.

The good news is that the model is symmetric, so only 50% of it needs to be worked on.
There are some teeth models in the library, but it hard to say if any are better or more suitable for whatever is planned.