OK, on the plus side of all of this...
You will formally be titled Joint Head of Legal I presume? That is great. That is a title that cannot be taken away from yoiu once you get it and is a stepping stone on your CV. Being an 'Acting Head' for 16(?) months prior to maternity leave doesn't - I think - entitle you to the role permanently (we need flowery to rule on this one as I think that doing it for less than 2 years prior to OML wouldn't be enough for you to claim it as 'yours'). It would have been so much worse for you to have been acting head and then not been offered the head role (either single or joint).
Being joint head of legal means that stresses are shared and responsibility is shared and the door is wide open to request flexi working. There are huge plusses. The negative is that I think more than anything you have slightly had your nose put out of joint and there is a risk that this person will fly above you and take the glory that you might otherwise have claimed. Now that must really really hurt, especially as you will be a bit out of sorts anyway, trying to re-establish relationships, worrying about DS, etc. And to make it worse, your new colleague is a 'he' and therefore might be able to wow all your superiors with his commitment and drive and elan, whilst you shuffle in all dowdy after a night without sleep, and shuffle out the door at 5.30 to pick DS up form childcare. I've been there, got the T-shirt.
Logically, you need to be able to simply accept that this is the way it works, it gives you lots of upsides, and fewer downsides. And they had you running the dept because they valued you, and they will still value you once you get back into the swing of it.
You are on a sticky wicket in terms of doing enything other than accepting with a smile and having a sniff in the loos now and then if you need to. The thing is it isn't 'him' that's the problem - what is (I think) causing the problem is the realisation that you are no longer the same career/sacrificing person you were before DS, and you will be seen by your bosses as being different because now you have loads of other commitments. That's what really stings, and I think this chap is just the physical embodiment of the fears you have regarding the career (which pre-DS meant the world to you I guess).
The thing where you do have grounds to be miffed is with regards to salary differential, but you need to look at it from the company point of view - the world would go crazy if every time they hired someone with a higher salary they then adjusted everybody else's salary. But they will need to harmonise it in future and I am sure they are well aware of it and it will smooth out over a year or two. Now is definately not the time to play the discrim card - it will only upset the company, and they don't seem to have done anything that illogical or unreasonable, yet. If they don't bring the salaries in line in the next year or two then you have much much clearer evidence of something being afoot than you do now.
You might want to speak to other wrinkly old city mums about the whole dealing with career post kids as it is sooooo difficult to reconcile the whole big career/perfect mum thing. There are lots of us on here and we slurp back a glass or two of vino quite often, so yell if you want to meet up and chat.