There is a serious problem with reintegration which I didn't consider before: the fees and block rewards will inevitably conflict between the different chains. Assuming B and C were discovered by different miners, the fee for transaction t3 in block B would conflict with the fee for the same transaction in block C, and the {B,C} set, while still representing only a single link in the chain, would have double the combined block reward.
I think the rewards can be split between the referenced blocks one way or another.
I considered that, but any such split would necessarily be after the fact; there are any number of ways blocks B and C, and possibly other blocks, could be recombined into block D, and then into block E, and so on. Reducing the reward after a valid block has been discovered doesn't seem very fair to the miners. Similarly, splitting transaction fees
ex post facto would introduce chaos into the miners' transaction selection process. It also seems like a much more involved protocol change than simply adding multiple parent blocks and merging their transactions.
Mining rewards mature after 120 blocks, and it seems reasonable if they change during that time. Though how to do this technically might be a challenge.