The good news is, you have less time to be annoyed by it

Personally, my belief in my own immortality only increases the older I get. Yes, Socrates died, but he clearly wasn't smart if he died. Me, on the other hand? I'm batting a thousand.

Would it kill you? Well, ultimately yes..

There is a deeper point here, not just pedantry. The point is that harm is a spectrum not a binary and one cannot meaningfully answer a question that assumes a binary.

(Tired quip “you must be new here”)

Yes, it is tiring. In this case, not really because it is at least for me, humorous in a Doc Martin sort of way. But elsewhere and on places like Reddit where the pedantry is often at best unjustified and at worst, wrong, it has made me spend less time on the sites.

Yes, but: depends on what he ingested. Deuterium/tritium cannot emit Alpha particles, the ones blocked by your outer/dead skin (also they're an order of magnitude more dangerous than the other at equivalent grey), so he's fine in that way (unless there is more in the pool, but that would be a bigger issue).

The issue with ingesting deuterium/tritium is that the dose will now have to be calculated/estimated per organ, and while I don't remember exactly how it's done, it's more complex than calculating the mSv he took (I can't give you more details, I'm not competent, I've observed a radiologist a week my bachelor year to decide which master I would do, I'm now working in software which should tell you everything)