I believe it would perfect for your bed.
no,you should use hard woods!
Yes you can just clean them with paint thinner if they have tree sap
For garage door frame you should use pressure treated lumber or composite material.
Yes, they will cut it to length.
Yes it will be ravaged.
The wood itself is strong enough. The dry wall is the week point. If the weight on the clothes rack is going to be under 100 lbs then I would suggest using 100 lb. Heavy Duty Self Drilling Drywall Anchors with Screws Model# 25220. If more than 100 lbs I would run a long board from the floor to the hanger rod so that the weight is transferred to the floor instead of the drywall.
The stores will usually cut their products to the lengths you need.