Thanks, Tim!

To clarify why I believe this is a bug...

The system does mark times outside of when the instructor is explicitly scheduled as unavailable, but only for that day of the week.

For days of the week where no availability interval is entered, all of the above applies.

There is no obvious reason why the system shouldn't assume they are not available on those days.

And it feels like too much of a blunder to have been built this way intentionally.

Huh...  It worked that way about a year ago when I was setting ours up, and had to stumble on the caveat to figure out why it was only working for some, not others.  I wonder when that changed.  Tossing this a vote.

In a similar vein...

Ideally, the monthly price adds up to more money the customer pays over annual or one-time prices.

So even if this has to remain hardcoded, monthly is generally preferable.

I know this is a few months old, and you may have figured this out already, but for the next person looking for this...

That feature already exists, though I agree it's not immediately obvious to new users.

When setting prices (or anything on the Event Setup tab), that bar at the top is where you set your defaults. 

The values do clear after you save the event, but in setting up CR for our club we found that the defaults did apply when adding new memberships later.

Caveat: This only works after you've used it at least once, and needs to be done for each event.  

TIP: Make your first event with whichever defaults will be the same for all events, then use Copy to make the rest, changing your defaults as needed.