When was Gandalf in Moria?

[Mänwe]

Formendacil, I permit you your amusement and my own. But is that really what it is saying? Allow me to be pedantic, it does state that he simply entered then left either!

"I passed the Dimril Gate..."

I take it as passing across the threshold of one entrance into Moria, it would strike me as odd indeed,

"To poke one's head through the gate and decide that it's a very nasty place,"

as you put it. Especially in Aragorn's case, he does not seem to me the character who would conduct such a casual observation; does he not to you, create a person who would undertake a task properly?

Yes although there is no specific quote stating he passed through, I do not expect anythin less from him. As why Tolkien does not simply have him say so. I would answer that it was in context of Aragorn's character at the time. He does not reveal an awful lot, he is in conflict with who he is meant to be.

For he is troubled to take lead even of the Fellowship when Gandalf falls from the bridge in Khazad-dum.

[Tuor of Gondolin]

That's an interesting theory about Strider not
going all the way to the West Gate. While it
may seem pedantic, it would square with his
letting Gandalf determine the way through
Moria. If he hadn't been all the way through
he wouldn't have Gandalf's firsthand knowledge.

Also, (somewhat erratically inspired?) foresight
seems to be a not uncommon occurence in even
the Third Age. Witness Eomer's response to
Aragorn:

'Thus we meet again, though all the hosts
of Mordor lay between us,' said Aragorn. Did I not say
so at the Hornburg?'
'So you spoke,' said Eomer, 'but hope oft
deceives, and I knew not then you were a man foresighted.'