Seriously, I don't think any of you are sufficiently alarmed about his behaviour.
Yes, & PP are not looking to be alarmist - just aware of 'the pattern' (see WA re: definiiton of coercive control) or "the script" (see Lundy Bancroft et al).
IceCream - do you feel that DD & possibly you are frightened of 'reporting' in case it angers SiL & he becomes (even more) retaliatory?
It is entirely understandable for you to feel something like "wait for for him to calm down & things will blow over" - but it is surrendering the only agency DD currently has (apart from hiding in hotels) - the ability to shine the light on the facts, & have a professional deal with them.
DD is trying to manage a situation she doesn't yet fully understand (because she is in it, & doing as best as she feels possible: better understanding will come only when she removes herself entirely from the toxic dynamic she is part of).
It may feel counter-intuitive, but the safest & wisest action is for her, or you, to contact Housing to say she is gone (& let the 93 day process start, & SiL respond to that as he will), contact the Welfare Officer to say she is currently Address Unknown, as SiL is experiencing difficulties, behaving irrationally & angrily - & must NOT know her whereabouts.
At that point she also needs to tell WO or CO about the threats he made to call her mental capacity into question, & relate what happened with the car/key throwing incident.
There must also, surely be a Women's Services Officer type person who she can talk to? By phone, not going to the base.
This bizarre & disturbing threat to blacken her name by pretending she is the crazy one is classic.
Not because it has any chance of working - but because it shows the degree of contempt & anger he is prepared to unleash on her.
What next - he carts her off the The Asylum, like a villain from a Gothic horror novel?
Can you see how disturbed his thinking is?
He is about to get blown wide open - he can no longer conceal some of his more nefarious actions from his parents now they know about the credit card copying etc. The military will rumble him sooner or later. He will be looking for somebody to be the recipient of all his "externalised shame" & by letting DD imagine she can handle the next 3 weeks solo, you are increasing the risk that it will be her.
Apologies for the strong words OP.
I don't want you to feel that you are wrong & being scolded, because you are doing your best in a very hard situation. You are managing so much already & I know you feel your hands are tied. It is also a thin line between not telling DD what to do, & getting through to her how precarious the next 3 weeks are.
Have a read of this for advice, to forward to DD, because the first 2 paragraphs certainly apply to her & she needs the heads-up - THIS is exactly what her H is doing, & the later paragraphs (obviously not the 'children' part, but the pattern is the same) are how he is likely to escalate. She needs to know that keeping her head down - while ostensibly a sensible course - is in fact anything but.
DD needs to hand this over to experts now - WO & CO at the camp, plus any womens/DV officer there, AND get herself a contact at WA who will act as a support officer to her personally.
speakoutloud.net/intimate-partner-abuse/post-separation-abuse