Bunraku (great name! Where is it from?). I am very sorry your PIM are saying these very hurtful and unhelpful things.
I think Roshbergosh has some great advice.
I am not yet an adoptive parent but I have been through the approval and we are now waiting and have a birth DD aged 9.
I think you will find that the adoption agency/county council you go through will be assessing you and not your in-laws, so ultimately they will want to know about you and will not be put off by your in-laws.
I think you will find if you stick to your guns with in-laws (so there is no sense of if they 'play up' they will be able to change your mind) they will come round.
It may help to explain to them why you have made this decision, if you feel able to, and how you see it impacting positively on your family, as well as your awareness of any possible problems and how you will combat them.
I am not sure how I would react to my mother-in-law crying! It depends, does she do this a lot, is it most effective to ignore it, or to comfort? I think if it were me I would try and be both comforting and assertive by saying "I'm so sorry you are feeling sad about this, it is actually a very happy thing for us, for our child who will have a sibling and for a new child who will get a forever family."
Try not to put any pressure on your in laws to bond with the idea because I agree with Middlesexmumy once little one is with you and you are a couple of years down the line then they will be just so happy to have a new little one to love they will do just that.
Personally, I would also see what things they do feel able to do to be involved - look after DC if appropriate for an evening when you have to go on prep course/parenting course or other training event, knit some clothes when little one appears or anything else you think is good.
I would also encourage your DH to be singing from the same song sheet (as I am sure he is) so your in-laws know this is a whole family decision and you are all happy about it.
I am sure you will make a great match for a lovely little one and I am sure your in laws will come round, and if they do not (I mean a few years done the line if they are still hostile) this will limit the amount of time they can spend with you as a family and I would (personally 0 IMHO) feel able to say that A few years down the line if the situation has not changed. But for now I would personally take their behaviour with a pinch of salt.
Oh, and I would not pick them to be referees!
Good luck.