Tax on Memberships is extremely wrong

On the heels of CRM-17417 we've had a couple of clients complaining about tax issues relating to CiviCRM memberships recently.

We've tested issues on 4.7.12 as follows - all tests are using a single tax rate of 10%:

• Using a contribution page to create an "online" membership seems to work okay - a new membership is created with the correct amount and subsequent offline renewals also work.
• Adding a membership using the "Add Membership" button and a price set results in an initial contribution that has the tax rate applied twice (e.g. $50 excluding tax becomes $60.50, not $55 as it should); subsequent renewals of this membership have the correct amount. (Appears fixed in master as of Feb 26)
• Doing the same without a price set appears to work.
• Adding the membership with a contribution in Pending state results in a Contribution that when edited exhibits multiple incorrect behaviours - on the contribution edit screen the subtotals have double the tax added (as in 20% instead of 10%), however the contribution total still shows the correct amount - attached edit_pending_membership_contribution.png demonstrates. Saving the contribution results in the total having the tax cumulatively re-applied to the post-tax amount (total is 121% of the tax exclusive amount).
• (Added 2016-12-13): When adding or renewing a membership through the admin interface ("Add Membership" button or "Renew" on a search result), if the amount is changed, the tax amount is applied from the original amount display instead of being calculated from the new amount - although this is not evident in the interface; it will show a contribution with the correct amount, but if you look at the line items via the API you'll see incorrect tax amounts.

Agileware reference 23973