Without knowing exactly what you mean by "senderLastMod", I'm can only speculate. I'm hoping it is something that provides replay resistance, because that would mean that you decided to keep "secure" at the cost of "useful" and "decentralized".
Yes, it prevents replaying. Don't keep us from your wisdom. Say why such efficient txs are not "useful" and why are "centralized"
Yup, a pony is an immature horse. Still more useful than a pegasus.
No. It is thing that has all requirements of pegasus but only pony features.