God, I'm very shy too and I'm getting nervous about walking down the isle as the bride, with everyone looking at me!!!
Personally, if I were a lone bridesmaid, I would feel much happier walking behind the bride and father, rather than going in first. That way most of the eyes will be on the bride anyway, rather than the bridesmaid, and also she'd be sort of "sheltered" by the people in front. Also give her a reasonable-sized bouquet to hold so that she can feel like she can hide behind it (I don't mean literally so enormous that it hides her, but just for the psychological effect) and making sure she has an outfit she's comfortable in – eg. for me it would be something that's not showing too much skin, possibly including a shrug or shawl or something, and heels that aren't too high for me to walk in, so I wouldn't have to be constantly worrying about tripping up.
I'd try asking her to see if any of those things would make her feel better, and if they don't, there's no reason at all why your mum couldn't walk with her, it's probably not very common but then again I'm sure someone somewhere has done it before and at the end of the day none of it is carved in stone, it's totally up to you.