I hope I have some good news for you having been through almost exactly the same thing myself (as Dad).
First, you are no longer a step parent - from this day forth you are a Bonus Parent ! Might sound silly now but if you stick to this, as time goes on, all kinds of things will subtly change. Labels are important, especially to kids.
Re: new baby - the ONLY important thing is to manage the feelings of DD so she knows she is not being "replaced" by the new baby. I would tell DD while she's with you, make her feel special that she's the first person to know, always, always use "brother/sister" and never "half brother/sister". Ask her what she thinks about this name or that name - in short, ensure her happy involvement, make sure she's more excited than worried - this will help as things progress and the baby starts to seriously make demands on time and attention. Then tell Ex over the phone the same day. What she thinks or says matters not a jot.
Re: money, talk to your lawyer but I don't think there is any mechanism for her to refuse the CSA calculation. If she wants to argue about it in court she will have to make, and pay for, that application herself and costs are only very, very rarely awarded in family cases. This wouldn't be one of them.
Don't worry about "tactics" because despite what you might hear or see on TV, actually, there are none (and it's really not the kind of thing a lawyer would say). The process is strictly governed by a tight set of rules. The only 'tactic' is either to co-operate, which is seen as being in the best interests of the child, or not.
If he agrees without fuss to the CSA calculation he is seen as reasonable and co-opertive by the mediators and this is the only offer he should make in mediation. Agreeing to the CSA calculation absolutely gives him the moral high ground and although he has historically paid more there is no legal or moral obligation for him to go on doing it once a formal calculation has been made by the CSA.
The mediators will suggest, firmly, that she accept the offer of agreeing to the CSA calculation and if she refuses that will be recorded by the mediator whose report can be used in court (should it ever get that far).
In the meantime, just pay the amount asked for by the calculation and keep the difference between the old payment and the new in a savings account. This money becomes either a legal war-chest, or, once things settle down, the price of a modern push chair !
If her lawyer writes nasty letters about money you have no worries - you are complying with the law and the spirit of mediation. Your lawyer will say that if she wishes to pursue it she can make an application to the court - but trust me, she won't.
Good luck with all of it.