I did find a previous post which mentioned phone-only customers from @BenMcr one of the forum VIPs and a VM employee
Message #25 here
https://community.virginmedia.com/t5/Tech-Chatter/Combining-phone-and-V6-line-in-one-modular-box/td-...
which mentions that the customer would get a hub but not have to use/pay for broadband.
Unfortunately, the link he quotes is the same link as I used above. In another recent topic (ref phone extensions), the VM forum team confirmed that the information contained there cannot be relied on. Sadly, another case of what VM say/do being two different things.
I think your difficulty is slightly compounded in so far as your existing package of TV/phone is not one which VM seem to promote and the last time I looked is disadvantageously price compared to TV/phone/BB.
As you say, it is a backdoor route to simply moving customers onto one of the bundled packages at higher cost.
Might be worth finding out from you mother which of the TV channels/services she actually makes use of. An alternative route might be to look at ditching VM and trying to put together an alternative package or using something like Freeview with an aerial or Freesat.
The lifeline alarm company should be able to offer some solutions to deal with the move away from POTS landlines. Many offer mobile technology as an alternative means of connection, inc. personal devices the user wears but the up-front cost for these can be high.
The emergency backup line which VM offer (999 calls only) is based on mobile technology like the ones at the link below. Devices such as the ones below
https://www.burnsidetelecom.com/terminals
allow access to the mobile network using fixed handsets. Some models allow a fail-over connection so that if the landline goes down, the device automatically switches to a mobile service. The lifeline company should be able to offer something with a similar.
I think if I was in your situation, I'd use the 18 months your price is fixed the same to investigate the alternatives and then give VM the boot and switch to an alternative provider or providers.