The price feed varies very little between one witness to another. If you mean when when do the revision, the price feed before and after the revision are the same. Only the constitution of the index varies. The algorithm compile the new index composition and built it with the last price recorded.
The memo key is the same as the account "announce" so everybody can see on the blockchain what is happening via Cryptofresh
Cryptofresh @announce : http://cryptofresh.com/u/announce
Cryptofresh @bittwenty.feed : http://cryptofresh.com/u/bittwenty.feed
The memo key is only used by witnesses to be able to read the data we send in the blockchain so they can built the price feed and adjust some parameter. You wouldn't need it to burn shares if you decided to.
Yeah, but for a theoretical MPA - if we were able to read the memo of a burn/null account then you could destroy some of the MPA to change an algorithm based asset's variables. Say destroying a few thousand $ of the MPA to increase/decrease the appreciation/depreciation.