There's two things going on here OP.
The hoarding, and the challenge to PiL way of thinking.
I work with patients, mainly older, after a fall, stroke, or other serious impact on mobility. Rehab to get home with package of care, or into residential home, etc. Others in the team will visit & assess patient's home. Hoarding will delay/prevent discharge home. Where patient is medically fit and insists in going home, we have to inform the fire brigade.
So just think that through for a minute. At every check so far with a midwife, health visitor, nurse, Dr related to your baby, you will have been asked 'do you have a working smoke alarm, battery and mains connected, in your home?' Every nursery and childminder will have to show they have fire safety training, and fire safety equipment as part of their certification.
And now imagine the conversation you'd be having with your health visitor, gp, nurse, midwife etc explaining why you put your baby in a known fire risk? Or the care of someone who chooses to live in that risk, withholding access to loved ones?
That is not a choice. It's a safeguarding decision. If you don't make it now, you'll find others make it for you.
It's also not a choice a truly loving or respectful father would make of his son.
That's my second point. You reference fil point about your husbands grandmother's hoarding. So this is 'the norm' for the family. Yet your husband has broken free. His father sees this as a rejection of their values, codes of behaviour. Yet he himself must know they have crossed a line, by refusing others access. His behaviour is erratic because his relationship with his son - who is challenging the status quo, standing up for himself, his wife, his own new family - is destroying your fils shaky authority.
In the past, cash has kept your dh in line. The classic 'all we've done for you'. Guilt tripping now re baby. Which will very soon become more 'All we've done for you'
Honestly, I think you've had a lucky escape for childcare. Find a better, long term solution. Suffer the pain now whilst everyone is, your employer will have to deal with it.
Time to define the boundaries for PiL. Protect your baby, don't let the weirdness go another generation, and help your dh continue to heal now he's started standing up for himself.