Not in this case. The seed given to the heir is actually one seed in a 1-of-2 multisig (this is why they need the descriptor--whatever wallet software they use to recover needs to be able to know how to find the coins), but it's a 1-of-2 where the primary key (held by you) can always spend and the heir's key (stored with this letter) can only spend coins if the primary key isn't used for a prespecified length of time. This is enforced by Bitcoin script at the consensus level, so not trusting a third party.

The advantage to this is exactly what you are getting at: an executor or your heir won't be able to spend your funds until you want them to (or stop using your key) even if have access to the seed/hardware signer/letter.

You are spot on. This is a really important point. At least with current pop understanding of Bitcoin, an heir who has no real idea about it may be very casual in their approach to recover (or just ignore it).