Don't worry, this is the right place for asking them. However, one basic feature of the Touhou canon is its intentional ambiguity, leaving most things to the readers' imagination. So only one question of yours has a definite answer.
Q: How does "time" in Gensokyo function?
The flow of time is the same in Gensokyo as in the outside world. Except for the Sealing Club stories in music CDs, which take place in near future, the majority of Touhou happens roughly in real time: the games often happen in the same year they are released, and the manga chapters are intentionally written to be in sync with real life seasons - this is why I strongly advise people to read them as they come out.
About the "stagnation" of the human village:
They are a small isolated population, and the only way they can obtain new technology is by learning from things drifted through the Border. For a village supposingly stuck in the 19th century, they are actually surprisingly advanced. ZUN said they had electricity, and an electrical lamp did appear in Forbidden Scrollery Chapter 1. More recently, the human village stage of Hopeless Masquerade showed villagers drinking from glass mugs, and a building with large glass windows. Those are all technologies the village couldn't possibly have when it became isolated.
Now, the other questions.
Q: Are youkai (excluding Ghost) in general considered as immortal in a sense - cannot die from age!?
A: Yes, generally youkai don't die from age; they only fade from existence when people no longer believe in them. In Symposium of Post-Mysticism, it is said that in the modern world, new youkai are still being born, because the world is still full of things unknown and uncomprehensible to humans (like "Murphy's law", or losing the TV remote control), but they will immediately disappear, because modern people are unable to affirm their belief in them, and shape them into legends that can be passed down.
Q: Whom do you think amongst all the residents of Gensokyo can go out?
A: The Border doesn't affect ordinary animals (Forbidden Scrollery Ch 6). It's also implied to not affect gods, since IIRC Kanako said she need to leave Gensokyo for certain businesses (I could be wrong about this one).
Q: Gensokyo barrier (radius)
A: A recent guess is 20 kilometers in radius. Gensokyo is a valley under Youkai Mountain, with a mythical peak higher than Mount Fuji.
Nothing is known about what happens when one go straight in one random direction.
Q: "Kamihakari" or "Kannazuki"
A: It's a good question, but the canon has said nothing about Kannazuki and Kamihakari (the meeting of gods) so far.
However, there happens to be a related tradition in real life...
In the whole Japan, only two places call October "Kamiarizuki" (the month where gods are present") instead of "Kannazuki". They are Izumo and Suwa. For Izumo, it's because the gods are gathering there; for Suwa, it's said because the Suwa god Take-minakata doesn't attend the meeting of gods. Local folklore says, this is because Take-minakata's body was too huge, so the gods asked him to not come.
Since Kanako is based on Take-minakata, perhaps she was not attending the meeting, even before moving into Gensokyo.
