Yes. All normal adjectives that go in front cause lenition. Not all modifiers do, e.g. gach, 'each'. This is normally classed as a pronoun in dictionaries, but it looks like an adjective to me when you say gach duine 'each person'. Whatever you call it, it doesn't cause lenition.
Nope, in this sentence you are specifying location, and not motion. The verb "meeting" is not present in the Gaelic; for that you'd need to use something like a' coinneachadh or a' tachairt :)
"I am meeting an old friend."
- Tha mi a' coinneachadh ri seann charaid.
- Tha mi a' tachairt ri seann charaid.