I have the Donation Plan on its own, and it shows on the Subscriptions tab - but there seems to be no way to donate and renew in ONE payment.

You can't renew a plan while purchasing any other plan. Plans must be renewed individually.

When someone goes to Renew it shows a BUTTON to renew (when clicked takes you to Paypal) and the checked donation is not included when you click Renew (you get just the renewal fee).

Correct, working as is designed.

How can the plans be setup so that they tally for first registration AND renewals?

Currently, you can't.

We are having a similar problem with subscriptions and merchandise being split into multiple payments. Did you get anywhere on this with subscriptions and donations in a single payment?

The only time you can purchase multiple plans together is if it is the first time the user is purchasing both plans. Afterwards they must renew each plan individually.