OpenTimestamps does - afaict each calendar only has one small tx per block as they are only submitting the merkle root of everything they're timestamping.
We don't need or rely on OTS for anything critical. This is just for fun ... OTS is way ahead of its time and we probably aren't the ideal end user.
That's correct. The four calendars are all funded by donations. You can see the donation addresses on the status pages of the calendars, eg mine: https://alice.btc.calendar.opentimestamps.org/ and https://bob.btc.calendar.opentimestamps.org/
To reinforce your point: "everything" really is everything. Via the magic of merkle trees the entire world can use a single transaction.
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