My second miner, while helping to build the second story on my garrison, dodged up a tree to try to duck some keas and is now stuck. Any hints on how to get him down?
Usually you can find a clear spot adjacent to a branch, where you can build an up/down staircase. Dwarves won't climb down out of a tree, but they will happily walk along the branches to a staircase and climb down that.
Is there any way to get civilians to wear armor? I don't want them wading into battle, they aren't trained, but I do want them to have the added protection of armor.
All except miners, woodcutters and hunters, yes. Just put them in a squad, and assign a uniform. Don't give that squad a barracks, or any orders (unless you want them to spend time training).
In 0.42 they will replace a cap with a metal helm (OH MY ELFING GOD YES FINALLY), but they won't replace shoes with boots, or gloves with gauntlets. This can be alleviated somewhat by modding in the [SHAPED] tag on shoes and gloves, or by using "replace clothing" rather than "over clothing", but the latter is a bit much for civilians.
Miners, woodcutters and hunters
can never ever ever be made to wear a helm while working, by any means whatsoever due to the longstanding "civilian uniform" bug. No workarounds. Nothing.