Having read your update about Christmas and Holidays OP, and how you and your DM will be feeling on the day without your Dear Dad.
So if DP thinks this would cause a serious / longstanding fallout with his family we need to have a rethink of the day.
He should be more worried about it causing a serious fall out with YOU. Since it will be hard not to resent in the future them taking control of such a small and initmate day and imposing their wishes on it. Its not like you were having a huge bash and making outrageous demands.
They have already taken control of Christmas and holidays at "considerable inconvenience" to you both. Now its the wedding. And every time it is at the threat of - if you don't do what they want - it will cause a family fall out.
Im sorry but it is your DP who needs a rethink. He needs to stop letting his "loud" family insist that their wishes are paramount. You have a child and you are building your own lives.
You need to explain to him that by constantly giving in to them, not being allowed to make your own decisions and have them respected that a family fall out is certainly in your futures. Neither of you sound like wilful, selfish or inconsiderate people so its not like your would make unreasonable decisions.
He needs to stand up for you because I can tell from your post that you are still brimming with grief for your dad and whilst we can cope with this kind of thing on a day to day basis - this is a key occasion which brings it all to the fore and you are clearly worried about your DM. I can appreciate that this is something that is difficult to debate when you feel like this with loud insistent people who don't listen.
Prior to this your DP's contribution has been to state that he "doesn't mind either way"... which is as weak as water and really is stepping back from all responsibility, almost saying that its not up to him but up to you. Allowing them to continue to push you, leaving it to you to manage his relatives with no input from him.. .so he can continue to be the good guy. I recognise this.
It is now up to DP to step up and present your case in a way that makes them stop imposing their will on you, on your DM and on your wedding in general.
It very much is up to him. Its not clear if your BIL and GF even know yet. So it appears to be MIL led. He needs to tell her to stop. Talk directly face to face to his brother and potentially GF when he invites them as a couple to the evening do, with all your closest friends etc. Emphasise that is the real celebration. He could have squashed this firmly by telling them - its all arranged and this is how WE are doing in. He is marrying you. Not his relatives.
Your DP needs to recognise how much stress this is causing you on top of your grief and how it is ruining the run up to your wedding which should be joyful occasion with your very nearest relatives happy for you both. Instead of this.
Why can't DH's relatives have a grain of consideration? The real celebration will be at your party. You've offered a very reasonable alternative.
I saw a pp earlier suggesting re naming the description of the two events on the invitations.Which I think is a very good idea. eg a party to celebrate DP and OP. or similar.
Wishing you both all the best x