radio silence for the weekend then a one liner reiterating that he can't move back in
At the present time the only 'one liner' that is permissible should be to the effect that you have nothing to add to your previous email and on no account should you initiate or engage in dialogue on other matters.
This isn't to say that you won't be entering into a limited amount of direct dialogue with him by emal before he returns, but at the moment it is vitally important for you to demonstrate to him that, much as you've discovered that you really didn't know him very well at all, he really doesn't know you and doesn't know what you're capable of.
You are in a unique position in that his absence from the UK has led to you currently holding all the cards and we will have to play them judiciously in order to achieve the best possible outcome for you and your dc.
I appreciate that this may go against your natural instinct to rant, rail, express your hurt and anger, and get him to agree to stay away from your home, but there are ways to achieve the latter without losing your dignity and your old school fil will hopefully prevail on him to do the decent thing.
To this end I suggest that you tell the leaking sieve your mil that after a most humiliating trip to the clap GUM clinic because he has put your sexual health at risk (subtext what kind of unprincipled cad does this to his devoted wife and mother of his dc) you feel that you can't be held responsible for what you might do if you see him this side of the afterlife next year, but that is unlikely to happen as you can't imagine that he will want or seek to return to your home as it would surely be anathema to him to live under the same roof as you when he is so madly in love and bound up with his ow.
Again, there is an element of what kind of unprincipled cad would move back in with his wife without so much as a by your leave after his adulterous behaviour has been found out? Surely a son of theirs would want to do the honourable thing and give the dw as much time and space as she wants and needs to come to terms with her discovery.
Hold firm, OP, and keep the fucker off-balance for the next week or so.