The swollen feet go along with the guess of heart failure..... VERY difficult for you to loose your pet, but at least it is nothing contagious, and you now know that the rest of tour flock is safe.
unfortunately, production RIR are wonderful birds, but not bred for a very long life.
I can't even guess what could be wrong with her, but considering her age, I would put her out of her misery.
Unless you see any symptoms in other other flock members I would chalk it up to age.
Sorry for...
a little dish of super small chick grit, or some Sweet PDZ (it you already have it on hand for poop shelves), or really fine sand....
They will of course dust bathe in the dish, and make a mess... but they will peck at some of it, and that will help reduce the pasty butt problems.
However, I...
you can post on Craigs list for free... and they have an animal section and a free section, great places to get rid of extra roosters
Also, if you have a local facebook page, those tend to be very active, and are also great ways to get rid of extra animals.
It depends on the individual chickens.... Usually a week, but it can take three weeks.
EXCEPT for the fact that we are now into fall...these super short days make many chickens stop laying, with laying picking up in the spring.
Be ULTRA careful with that heat lamp.... Lots of coop fires.