Tried to PM you back but think your box must be full! then found this thread so yo can have my response here, but feel free to PM me again if you want to clarify anything.
A bit about my experiance, we had to have our old boy PTS just before christmas

unfortunatly old age had just caught up with him and his body would no longer co-operate with his brain

Phoenix was well into his 30's and had cushings and almost no back teeth left, he also had terrible worm damage from before we got him so keeping the weight on him was pretty hard in the last couple of years, so I got to try lots of things and learnt alot from it! and nutrition is something I am really pasionate about.
Alot of what will work will have to found on trial and error, 'cause depending on what else is going on and the individual some things will work, others won't, its a real pain.
I would start by getting rid of the mollychop, it really is just a bulker and has no real nutritional value, it is just chopped straw covered in molasses (sugars) to make it palatable.
Switch that out for some alfalfa chop - Alfa A/lite/oil by dengie is readily available, but if you can get some Lucibix from
Simple systems they are really good, it is pure alfalfa (no mollasses added) compressed into handy chunks which you just soak for 10 mins, this makes them easily digestable and very easy for oldies to manage

How does your lass manage with feeds? does she drop a little/lot as he eats? If you find she does you may find switching to a cube rather than a mix and soaking it lightly will make it easier for her and TBH that will make them more digestable so she should get more benifit from an equivilant ammount of feed. I really like Baileys No. 4 top line cubes, I found they were effective and also a very fair price

I have also used No 1 in the past but again can't get it round here very easily.
I would stick with the alfabeet, its great stuff, it was difficult for me to get it locally so I used speedy beet and grass nuts all soaked together for the same effect.
I also had Phoenix on barley, it is fantastic for weight gain but introduce it slowly, it can make some a bit hyper
I also fed oil alot, it is really easily digestable and gives lots of calories but won't hype them up

a cheap and pretty effective option is regular veggie oil from tesco - cheap and easy to get! even more effective is soya oil but we could never find a regular supply! I found the best of all is Linseed, you can get the oil but I found it was expensive that way, getting ground linseed is much more cost effective, I get
this one (2nd one down) (simple systems do linseed aswell) and the tub lasts for ages, and it is really easy to feed.
Another way to make the most of the feeds is definatly to feed a probiotic, I used
Biotal Equine Gold
Above all else it is absolutely essential that you ensure she is getting adequate vitamins and minerals, oldies need different things to younger horses, the easiest way would be to buy a specially formulated old folk feed or supplement but depending on what else you are feeding and how much of it you might not need to do that

it is also essential that she gets plenty of really good quality forage all the time.
Re rugging, I totally agree with keeping rugs on if she is cold, but if she is warm then keeping her rugged could well cause weightloss - they can sweat weight off as well as shiver it off! Also if she has any arthritis you may find she isn't grazing as much during the day if she is stiff in the mornings, which standing in can do, sometimes being out can keep them more mobile which means they are more effective at grazing. Its a nightmare with this stupid weather at the moment though, who knows what it will do next!
If you could give us more specifics about your horse, like her type, height, current weight, her character, how good the grass is where you are etc etc then we might be able to be more specific on what might suit her, also do you have any pictures and have you
condition scored her?