Pre Owned Asset - GROB election

I have a situation where due to Mum’s Multiple Sclerosis care requirements, a Daughter and Son in Law and a Mum and Dad each sold their respective properties. A replacement property was bought in the names of the Daughter and Son in Law only. Mum and Dad gifted £260K (50%) towards the cost of the residence as they needed to all live together. Mum is in receipt of PIP and carers allowance and pays the carers allowance across to her daughter.
Despite the fact that this has not been done for any IHT avoidance, it would seem that they are in POAT territory and will have been so since 2021. Based on the value of their assets and income it would however make sense for them to elect for the matter to be treated as GROB.
We are 3 years down the line; does anyone have any experience of dealing with this situation with HMRC as I’d like to know how rigid HMRC might be in relation to their rules, timings and treatment so that I may advise my clients of what to expect!
I appreciate your thoughts on this.

Might there also be an argument that it is a GROB anyway (despite being a gift of cash) - either due to the proximity of the payment and purchase or under associated operations. Particularly if M+D provided the cash direct to the solicitors acting on the purchase and/or were clients in the transaction.