To determine if the entire surface of a board that you've run through a thickness planer is flat try the following: Scribble a chalk mark on the surface of the board. Then run it through the planer until the chalk mark disappears.
Warm vinegar will generally soften the most stubborn old glue on old furniture. Dipping the parts to be reglued in warm water, and then letting them dry out completely, will help to open up the wood pores and allow the new glue to enter the wood more freely. Warming the parts on top of a heater or in the sun will also help open up the pores of the old wood before regluing.