It appears that more people are interested in sharp-nosed leeches than the fate of a diabetic grandfather 
Well, if he is indeed alive and still suffering from diabetes, the observer would have said exactly the same thing, for
I didn't know Guinevere's grandpa is a diabetic
would violate verb tense consistency. Guinevere's grandpa is rather upset for the insinuation of his death!
Now if Guinevere said
I didn't know Dad was a smoker.
Does it mean he still smokes? Is he dead? 
I suspect that you may find the following:
Thal thinks that this post was far too long for its own good.
Er, no...
Thal thought that this post was far too long for its own good.
...or is it
Thal thinks / thought that this post is far too long for its own good
?
Discuss.
In the meantime...
"my once diabetic grandfather was interested in sharp-nosed leeches"
OK, so was my grandfather diabetic only once? (i.e did he contract and suffer from it, get cured of it and then never suffer from it again?) - was his interest in those particular kinds of leech a thing of the past or does it continue to this day? - or even was his diabetes overcome by that interest?...
Er - I've got a better one.
"This thread has outlived any usefulness that it might ever have been thought to have"...
or is it
"This thread has outlived any usefulness that it might ever have been thought to have had"...
...(yawn)...
Best,
Alistair