Simitra--
I've asked around, and the best thing to do seems to be the simplest -- as you suggest, phone the airline beforehand and explain the situation (better than trying to email). It may be appropriate to offer to email or fax through your mother's itinerary, including flight details, once you've made contact with a human -- having bits of paper floating around the system seems beneficial.
If your mother presents herself to the airline desk in plenty of time, there are lots of things they can do to reduce her need to queue -- including letting the cabin crew know of her requirements. A good crew will make sure that she gets the attention and help she needs without making her feel bad about it. You might consider writing a letter explaining her situation for her present to people as and when: this can be less awkward. But that depends on her... unfortunately, if she isn't comfortable about letting people know there's not a lot that can be done after she gets off the plane at the far end.
My mother's in a similar situation, as she's had a hip operation and travels a lot. At first, she was very reluctant to be thought of as 'disabled'... but once she'd found out how much easier things are if she let the system take care of her, she's now firmly in favour of making the most of every break available! Another elderly aunt had a favoured car service, where the driver not only took her to the airport but looked after her until she was safely ensconsed in the system - this may or may not be an option.
Sorry I couldn't be more specific. Do let me know how it pans out: in one of my other lives, I have an interest in the experiences of disadvantaged consumers.
R