Has she been treated for coccidia? Green scours can be due to coccidia, and having been weakened due to having lambs later in life (not the oldest, I knew a ewe who raised a single lamb at 16!) this may have caused a problem, where healthy ewes often are not bothered by coccidia.
The lamb may well have had infections because the ewe had something that passed across the placenta/in the milk.
Did the second lamb die after birth, or was it stillborn? May it have had an infection or rotted inside the ewe?
Another thing that can cause scours in an older ewe is Johne's disease. If this is what it is, it can be diagnosed by a combination of blood and faecal tests (they are fairly insensitive tests, so a combination will give you the best answers). There is unfortunately no cure, and affected animals should be culled. They pick it up from faeces of affected adults while they are lambs, and it has a long incubation period, so they are over 2 years before they show signs, often later.
Worth continuing with all the symptomatic treatments you have mentioned, the likes of the pro-rumen will give nutrients, vitamins, probiotics, and administered with water, keep the hydration up.
TLC is also good for invalids, and hopefully will pull her through, but after a few days, it may be time to make a difficult decision, with her age and signs, she may not rally, and you may have to relieve her suffering.
Hope she feels better
Suzanne