Re. your seat, I suspect someone who knows what to do would be able to get that saddle into usable condition; not usable for century rides, but good enough for the purpose you describe. I won't pretend to be the guy who knows what to do, but no doubt the first step is to get some oil into the fibers.

Re. the Benelux, same thing, more or less; are you sure it needs to come apart? Get some penetrating oil in there, and let it do its job, then apply gentle force. What's the derailleur made of? If there's no brass in it, you may be able to soak the whole derailleur in a molasses water solution to dissolve the rust; then dry it and apply penetrating oil....

Re. the cables, same thing, almost. First get some oil in there, then get the cables moving. It may not be necessary to replace anything, but it shouldn't be hard to replace the inner wire if necessary. If the housing is bent, straighten it carefully, by hand. If it pinches the inner wire, you may be able to unpinch it with pliers. You're missing a couple clips to hold the housing to the frame, especially the down tube; those will help a lot.

I have a ca 1951 Norman, very rusty, the brake cables on which were frayed and completely frozen when I got it; oil and gentle force got them working, but I still can't get the nuts on the caliper loose, so haven't replaced the cables-- but the brakes work fine!