2 - assuming a help point is situated correctly, contact using that & a taxi would be supplied, either to final destination or nearest station that wiuld allow connection into service required.
For the number of likely users, that is likely to be sufficuent provision
There is a help point at Chippenham. Not sure how said taxi would collect a passenger dropped off the train and stuck on the island platform if the lift was broken though, which was the concern being expressed, now that the barrow crossing isn't available.
Taxi solution semi-works for people catching late night trains out; I believe there are 2 available in the Chippenham area on Cabfind, though anyone being put into a taxi because they can't board a late night train will find they're later than intended at wherever the can takes them - so if they're catching the last London train, a drop off at Swindon could well be after the last London train has left there ...
Frankly, the solution is to keep the lift in order; the issue causing the concern was if (1) the lift and failed and (2) the train manager failed to realise this as he helped the wheelchair off the train. So it's remote. The risk of harm to a wheelchair user dropped off (but still able to use the help point, unless you have a third failure) isn't a danger, but a risk of irritation - so the overall answer to a risk evaluation is "be aware, don't panic, reassure the customer" and not "lets spend a lot more money on a backup system of some sort".
