> What do you mean by monotone in the context of sponsor transactions?

I take this to mean that the validity of a sponsor txn is
"monotonically" true at any point after the inclusion of the sponsored
txn in a block.

> And when you say tx-index, do you mean an index for looking up a
> transaction by its ID? Is that not already something nodes do?

Indeed, not all nodes have this ability. Each bitcoind node has a map
of unspent coins which can be referenced by outpoint i.e.(txid, index),
but the same isn't true for all historical transactions. I
(embarrassingly) forgot this in the prior post.

The map of (txid -> transaction) for all time is a separate index that
must be enabled via the `-txindex=1` flag; it isn't enabled by default
because it isn't required for consensus and its growth is unbounded.

> > The current consensus threshold for transactions to become invalid
> > is a 100 block reorg
>
> What do you mean by this? The only 100 block period I'm aware of is
> the coinbase cooldown period.

If there were a reorg deeper than 100 blocks, it would permanently
invalidate any transactions spending the recently-matured coinbase
subsidy in any block between $new_reorg_tip and ($former_tip_height -
100). These invalidated spends would not be able to be reorganized
into a new replacement chain.

How this differs in practice or principle from a "regular" double-spend
via reorg I'll leave for another message. I'm not sure that I understand
that myself. Personally I think if we hit a >100 block reorg, we've got
bigger issues than coinbase invalidation.