You do not want a fall out but this really has already happened because of her own behaviours towards you. She has acted unreasonably throughout towards you aside from her hoarding issues.
I think your H is deeply ashamed of his mother's hoarding and has lived with her hoarding problems for many years. Hoarding often starts in adolescence and becomes worse through adulthood.
She will have to be told by your H that a visit to her house is not possible. End of. She will not like it regardless and will throw her toys out of the pram but your health and safety now is of far more importance particularly as you are pregnant.
From the International OCDFoundation on the subject of Hoarding:-
"Hoarding usually begins early in life, though onset can vary greatly. It can occur in children, and we have seen it as young as 3-years old. For young children, hoarding may look different because parents control what children can buy and the level of clutter in their rooms. More apparent in children are extremely intense attachments to objects and the tendency to personify things, applying human-like characteristics to objects. In addition, children who hoard seem to have more difficulty recognizing hoarding behaviour as a problem.
The typical age of onset for hoarding behaviour (though not hoarding disorder) is around age 13. At that time the behaviour is usually mild and would not be considered a disorder. Hoarding typically progresses to become a moderate problem in the 20’s and 30’s, and a severe problem in the 40’s and 50’s. Onset appears to be earlier in women than in men, though hoarding occurs more frequently in men than in women. Late onset of hoarding (after age 40) is rare and seems to occur in people who have mild hoarding to begin with and suffer a loss of some kind. Most people who hoard describe a chronic course, while a small number describe an increasing or fluctuating one. Stressful and traumatic events are common in people who hoard and may be associated with periods of worsening symptoms. Recent epidemiological studies suggest that as many as 1 in 20 people have significant hoarding problems.
Hoarding is a complex disorder that is believed to be associated with 4 underlying characteristics. First there are certain core vulnerabilities including emotional dysregulation in the form of depression or anxiety along with family histories of hoarding and generally high levels of perfectionism. Second, people who hoard appear to have difficulties processing information. In particular, these difficulties appear as problems in attention (including ADHD-like symptoms), memory, categorization, and decision-making. The areas of the brain that control these functions roughly correspond to the brain regions that have been shown to activate differently in people who hoard. Third, people who hoard form intense emotional attachments to a wider variety of objects than do people who don’t hoard. These attachments take the form of attaching human-like qualities to inanimate objects, feeling grief at the prospect of getting rid of objects, and deriving a sense of safety from being surrounded by possessions. Fourth, people who hoard often hold beliefs about the necessity of not wasting objects or losing opportunities that are represented by objects. Additional beliefs about the necessity of saving things to facilitate memory and appreciation of the aesthetic beauty of objects contribute to the problem".
Apologies for wrongly attributing the other comment to you, had some crossed wires there!. Again sorry.
Your MIL also likely has other serious mental health problems associated with hoarding (I mentioned anxiety and depression but that only scratches the surface really); this lady cannot be reasonable even if hoarding was not an issue. She does not like you and cannot cope with you visiting. If anything of hers is moved she will have a right old go at the two of you.
It is not your fault that she is this way. I also think your DH is a big part of the problem here along with his mother. And I do not think you can do much about him either. He grew up with hoarding after all.
Again from the OCDFoundation":- (as US based website on the subject of hoarding)
"Adult children often have a very strained relationship with their hoarding parent. As adult children move out of the home, they may become estranged from their hoarding relative due to disagreements about how hoarding should be handled. Adult children may also blame the parent for the condition in which they were forced to live as a child. As these children marry and have children of their own, they are most likely resistant to ever bringing their children over to their parent’s home. They are embarrassed and would not like their children to model the hoarding behaviours they see. Therefore, grandparents may be isolated from their grandchildren as hoarding may be seen as a bad influence. Not only does this create distance in the family, but the person who hoards becomes even more isolated. Adult children often copy or oppose the behaviour that they witnessed as a child. Either hoarding behaviours are learned and repeated, despite living separately, or the adult child, embarrassed and disgusted at how they lived, keep next to nothing. For example, if a daughter has observed her mother’s hoarding early in life, and then moves out, she may be more likely to develop her own hoarding problem as a result of watching her mother. In addition, if a divorce resulted due to the hoarding, adult children may blame the break up of their family on the person who hoarded. They may have been taken away from their parent, causing feelings of abandonment, as though inanimate objects meant more to their parents than they did. This causes significant psychological distress and often impacts their future relationship behaviours.
Not only do the affected family members suffer the physical and emotional effects of hoarding, but so does the person who hoards. For example, the person who hoards may resent loved ones who offer advice, but little help. Those who live alone may resent family members that stay away. Extended family members may feel shame related to the hoarding problem in the family and will keep the person who hoards from the rest of the family".