How to make a membership renewal form pre-filled with last year's values?
I am not commenting on the key part of your question (too late at night) but just want to be sure that you are aware of ability to send out 'checksum' in your reminder emails, so when the link is clicked by the renewing member they arrive at a Contribution/membership page and the user is temporarily logged in so the form pre-fills. That way the system knows who they are and will treat them as an existing member who is trying to renew. It won't, afaik, show how much they paid last time, but it will default to their existing Membership Type - which might be the same thing depending on how you set up your Price Set and Membership Types. Good luck