In the worst case; you can take the BB out, run a 1/2-13 or M12 threaded rod down thru the seat post, put a washer and nut on thru the BB, and pull the seat post out. Use a LOT of lubricant on the pulling end of the threaded rod, and plan on scrapping the threaded rod - unless it is high grade. This sort of tool can exert over 10K lbs (45KN).

Canti brakes pads: https://www.amazon.com/Kool-Stop-Bic.../dp/B000GDCXBA will improve the braking performance. Also read: https://www.sheldonbrown.com/cantilever-geometry.html