It's not a permitted route, however if you explain your accessibility requirements to West Midlands Trains, they should say which alternative arrangements and reasonable adjustments they suggest. For example, that could mean a taxi from Lichfield Trent Valley, or allowing you to travel via Birmingham where that wouldn't normally be permitted.
Of course when you intend to travel on a route that is not permitted, you can always use split tickets, e.g. Stoke-on-Trent - Birmingham New Street and Birmingham New Street - Lichfield City, but why can you not travel via Lichfield Trent Valley? I recall it having lifts the last time I changed there, although I didn’t use them and I would generally try to avoid changing at Birmingham New Street if at all possible.
Good point - lifts were indeed installed around 2020. In which case the OP's mother should not struggle to change at Lichfield TV.
Lichfield is a lovely little city to visit! But as you say, Platform 3 at Trent Valley is currently out of use as per the West Midlands Railway website.
Welcome to the forum. The arrangements they describe until the platform is repaired appear perfectly reasonable - does your local knowledge show otherwise?
