If I set up a plan for the current calendar year and someone signs up on June 15th - will they be charged 1/2 of the full year's amount?

No, the user will be charged the full amount regardless of what point in time of the subscription.

then how do we work a scenario where people sign up for a calendar year - but when someone joins in the middle of the year - they don't pay the whole amount.

You can't, there's no prorating for subscription durations.

12 month subscriptions - therefore prorated, manually reset the subscription date back to January 1? I don't see any other way - but a ton of work that would be nice to not have to do.

Don't understand, changing the subscription date won't refund any amount or anything of the sort. I suppose on request if using Paypal you could refund the user half their purchase amount. You could after 6 months disable subscription to Plan A (full price) and enable subscriptions to Plan B (half) price; this would give the reduced price due to subscribing mid-way through, but takes about 2 clicks every 6 months (could easily automate it with a cron job or something of the sort).

from what you're saying - there's just no way to do this

This is correct. Best approach I suppose is to create multiple plans and enable/disable them based on the month. You could even automate this using database queries within a new CB plugin utilizing triggers or professional subscriber incubator project CB Queries to fire queries at various CB triggers.

You could also give users "bonus" time for subscribing say 1 month before expiring that will carry over to next year (next year they'd still expire on say January, but maybe they get say 3 months free for next year). Bonus time is only available when using "this XYZ" type durations.