>> 2. (from Suhas) "once a valid transaction is created, it should not become invalid later on unless the inputs are double-spent."

> This doesn't seem like a huge concern to me 

I agree that this shouldn't be a concern. In fact, I've asked numerous people in numerous places what practical downside there is to transactions that become invalid, and I've heard basically radio silence other than one off hand remark by satoshi at the dawn of time which didn't seem to me to have good reasoning. I haven't seen any downside whatsoever of transactions that can become invalid for anyone waiting the standard 6 confirmations - the reorg risks only exists for people not waiting for standard finalization. So I don't think we should consider that aspect of a sponsorship transaction that can only be mined with the transaction it sponsors to be a problem unless a specific practical problem case can be identified. Even if a significant such case was identified, an easy solution would be to simply allow sponsorship transactions to be mined on or after the sponsored transaction is mined.