The current changes only impact storage of non-active voting and payout info (old votes and payout information is no longer retained in hived after a post has paid out).

The actual calculation of payouts still needs to be done in the 1st layer code (the blockchain code itself), as you believed previously. The only way to move this calculation out of hived would be to pay the rewards in something other than HIVE/HBD.

As to the reasoning for the 7 day window, we haven't looked into why it was set to one week, although I was under the impression (seemingly like you) that it was to prevent too much load on hived (the core blockchain process).