I have not read all of the posts so I apologise if my comments are irrelevant or have been said already.
You social services appear to be preparing for plan A and plan B so they may also have concerns but are bound by law, Human Rights etc and if they don't do that the case will be taken from them.
I agree with the comments above, for example by Lucky Legs, and hope that her suggestions help. I can't help wondering how a young man with, presumably, no experience of autism, is going to cope with living with your GS. He may surprise you, or he may deliver an ultimatum to your daughter. Even your daughter is going to find it hard to keep to the strict routine your GS needs. But from the point of view of the social services, if your daughter is determined to try they have to give her a chance and not assume she will fail. She will soon find that the novelty of the room etc will soon wear off for your GS and are no substitute for time, care and patience. She may be pleading with you after a few months to take him back.
Therefore I suggest that the most important thing for you to do is to be encouraging, show your support, do not throw up barriers while remaining realistic about the risks, so that your daughter will not be afraid to turn to you when she needs help. Do whatever you can to help her, be encouraging, let her know that you will always be there at the drop of a hat, give her no reason to think you tried to stop her having her son, or will say 'I told you so'. I do realise that this will be very hard to do and it will be hard to guard against the obvious risks,but the more time she spends with her son will make her realise how hard it will be. It will also be much better for GS to see you all in harmony so that he does not feel that you are fighting over him.
A hair test will show the truth of whether or not she is drinking,try not to pre judge it - if Social Services will not pay you may be able to get it done privately with a little subterfuge (only as a last resort, but it might put your mind at rest, or prove you were right). As for cannabis, although I have never tried it, it is legal in many countries and might be one day in the UK - unfortunately there are worse legal highs easily available.
You have my heartfelt wishes that this will all turn out well,there will certainly be tears to come for some of you as there is no easy answer to this situation. You may see years of hard work and love go down the drain, but I sincerely hope not and hope that your GS, the innocent victim in this will in the long term, emerge unscathed. You should be very proud of what you have achieved.