Hmmm, not sure why they'd stop working in your case. I have some questions to help zone in on the issue. Are you on the main version or beta? Are the jobs being created? Do you see little blueprints in the field? Are they red and have an error explaining why they can't start? What does the production window (K) show? Any red text with some sort of error message?
The most common cause of them not doing the jobs are that they have the required skill disabled, or they are too busy to do the jobs (doing something else).
Yes, arboretums are exactly like farms, but with trees. You need the Forester for that room, though. Or, at least, someone with the Forestry skill enabled.
Wood beams should block pathing just like walls. It's just a different texture, really.