question 1: if you buy an insert, or use a car seat in trailer, then you can start pretty much from birth. Otherwise, once they can sit up.
question 2: can't help there. maybe she'll be won over by the option of staying home in peace and quiet while you take the wee one out?
my experience has been that drivers will avoid you like the plague if you are pulling a kiddie trailer. huge wide overtakes. and the trailers themselves are terrifically stable and safe.