Hi Pippa,
Yes, you have found the answer to your own question! It is too early to try and reserve her seat for travel in April. Given her age I would suggest that you do what I do for my parents now - book her tickets on-line and then when you get them, phone the relevant train operator/s direct and not only reserve her seats but request assistance for her. Then if someone is sitting in 'her' seat there will be a staff member with her who will also have the authority to ask them to move.
I've done this ever since I witnessed the scenario that Helen T warns you about! An elderly woman travelling on her own was accompanied onto the train by a member of the station staff only to find that her reserved seat was occupied. At first the offending passenger refused to move and the staff member pointed out to them that the train couldn't leave until he got off, and he wasn't going to get off until he'd safely seated her. Every minute they sat tight was another minute that would be added onto an already delayed train journey. He refused to countenance the suggestion that there were surely unoccupied seats elsewhere he could put her in on the grounds that this was the seat the passenger assistance staff at her journey's end would expect to find her in, in order to help with her luggage etc when she needed to disembark. Making them look elsewhere for her ran the risk of either them not being able to find her and her being left to fend for herself or else the train would yet again be delayed whilst they hunted through the train for her. In the end the mutters and increasing restlessness of the other passengers shamed the offender into moving.
When faced with this situation on my own, I do usually find that a polite request to let me occupy my reserved seat is all that's needed. However, if that doesn't work and there are no nearby unoccupied, unreserved seats nearby, then my usual strategy is then to simply stand right alongside them, boxing them in by putting my handluggage or small case right by their feet (or even better precariously balanced on the luggage rack right above their head!) or, if it's a table seat to actually place my bags 'out of the way' on the table! And to then make sure that everytime somebody needs to get past that I lean right into them, (even better if they happen to be holding a cup of coffee/tea at the time!) so that people can get up and down the aisle etc. Rarely fails to make them decide that it would be better for them to try and find another seat and let me have mine but if it doesn't at least I know they have had as uncomfortable a journey as I can possibly make it
SM
SM