No one here can diagnose that the mum is not depressed, equally we can't say that she is depressed, but it does sound as if this is a possibility
OP you obviously can't have her visit but if you live near enough no reason why you couldn't meet 2 m apart for a wall in the park, My daughter visits, but doesn't come into the house and I keep over 2 mapart and sanitise things she touches, if your little lad was there that would not be easy, and you don't want him to be upset, a tantrum is not a sign of trying to control you, it's a sign that your little one has become overwhelmed by his feelings, and needs help to regulate his emotions, the sort of tantrum that much older children and adults can have is a very different thing, [although they can also have an overwhelmed tantrum]a toddler doesn't have a developed enough brain to do that, neuroscience and child development specialists acknowledge that, so just don't listen if anyone tries to tell you out of date stuff like that. Littleun won't understand, so that would be risky, what about using a back carry, IF you think your mum would stay away.
If you think your mum is depressed, that is actually an illness, and she would benefit from medical help, the WHO do consider that dealing with mental health issues is part of dealing with the pandemic. Does your mum see a nurse for any of her health needs, if so maybe letting the nurse know of your worries, might be a good idea, or dropping the doc a note with your worries, if she is on medication she will need reviews, and maybe that might be an opportunity to ask about her wellbeing, or ask age UK for advice, you will not be told of the outcomes of any such intervention though. depression makes life hard, and sometimes to help people have to be a bit proactive help others, and now is not an easy time to do so.
Siblings have their own opinions and do things differently, and you must do what you feel is right, harder with your husband, but we are not living in the 50s and I'm sure that if either of his parents is still alive you'd not be trying to tell him what to do unless it risked you and your child's safety.