How to remove these windows for cleaning

There doesn't appear to be too much holding them in at the best of times. Unless you want to replace the entire window you should look at getting the top and bottom sills (or whatever they're called where you are) straight. From your description I would say that there is a pretty significant difference in the opening when measured at the edges and the middle. That shouldn't be the case at all.

Remove the trim from at least the top and bottom of the window frame and shim the top and bottom as required until you have a nice even distance all the way across. Then replace the trim and touch up the paintwork as required.

The Family Handyman has an article on How to Install Craftsman Trim, which says to simply glue and nail the stool to the jamb using 2 1/2" finish nails.

Once you have the stool cut to length, add a bit of wood glue to the length that will sit against the window frame. Position the stool where you want it, and drill pilot holes through the stool and into the jamb using a 5/64 drill bit (using a clamp to hold the stool in place can be useful). Next, fit 8d finish nails through the stool and into the jamb. Install the apron tight up against the bottom of the stool, making sure to nail into the framing when attaching it.

You could install a more substantial apron, and additionally nail through the stool and into the apron. This will add a bit more support for the stool (especially if the apron is attached to the framing), but may or may not fit aesthetically with the rest of the trim.

Obviously with this method you'll not be able to do chin-ups from the stool. But once the nail holes are filled and the piece is finished, it should look nice for years to come.