@TheOchreEagle This is a really difficult one. Mentally I would be running a mile, emotionally I would worry about how the young couple would cope.
And I think there are two issues here, The welfare of the baby and whether the grandparents' home should become the primary support arrangement. It seems to me you are NOT saying "I don't care about my son or grandchild" but you are saying you work from home, already have the care of your son and mother, plus work fulltime and have a partner, a partner who is very resistant to the idea. Your son appears resistant to guidance and both young parents have learning disabilities. Social services have a poor track record of providing timely support.
Taking them in could damage your relationship, not just with your partner and son but also affect your livelihood. Those are very legitimate concerns.
Where I'd differ slightly is on the "they got pregnant intentionally". Even if the pregnancy was planned, autism and learning disabilities can affect judgement, understanding of consequences, executive functioning and parenting capacity. That doesn't automatically mean they can't be parents, but it may explain why social services are involved and why support is being discussed.
What jumps out at me is that social services are not asking you to babysit occasionally. They are effectively asking you to become part of a supported-living arrangement for an indefinite period (possibly life). That's a huge commitment. Once a baby arrives, "half the week" can very easily become more, especially if the young couple struggle. I'd also be cautious about relying on: promises of future support from social services, verbal assurances that the arrangement is temporary, assumptions that the girlfriend's family will continue providing half the support. People often agree to things under pressure and then find themselves unable to step back later. And that person maybe you.
From the baby's perspective, there's also an argument that a stable, planned arrangement is better than being shuttled between households if everyone is already predicting conflict and stress.
OP, don't agree out of guilt, before you make any decisions, ask social services for a written support plan, ask what happens if you say no and ask what accommodation and parenting support are available independently of your home. Give allows you to protect your relationship and employment as well as considering the needs of the young couple.
My Aunt used to be a foster parent and at one point she had vulnerable adults and their newborn babies in her home to guide them and then offer support when they got their own place in the longrun. SS were assessing if they were capable on their own to be parents and they were deciding if they could do it. And this is the thing, if the parents realise its too much and they get overwhelmed as autistic people often do, who will step in? In what they propose I think it will be the grandparents. You know what your son is like, how difficult will it be with a baby for him to care for?
I have an Autistic brother and he does not fair well around kids let alone babies and he is a professional. But I also have an autistic cousin and she has three kids and is a great mum - the devil is in the detail I am afraid.
In truth, reading that post, my instinct is that you've already decided it won't work and are looking for permission to say no. And sometimes "I love you, but I cannot have you all living here" is a perfectly reasonable boundary, but its easier said than done.
The part that worries me most is actually the partner. If he's already "not at all happy" before they move in, bringing two vulnerable young adults and a newborn into the house sounds like a recipe for resentment, rows and stress for everyone involved. Saying yes to avoid feeling guilty could end up making the situation worse rather than better.