No, you are not being oversensitive.
IT would not surprise me if they are having a flap behind the scenes and getting legal advice OR have now received legal advice and are wondering how they can 'fix' the fact they didn't consider you from promotion. They could find a number of ways of wriggling out of it, but it would depend entirely on the exact circs.
They may have been curt with you initially because they may have been wary about your motives - your emails about employment pay and then about promotion may have had them thinking you are more keen on getting them over a barrel than on getting back to work. (Not saying that you have done anythign wrong or that you shouldn't be dong any of this, mind, I am merely speculating on why they are being like this)
Anyways, there may be loads of reasons for this, but what is clear is that they are NOT acting in the most welcoming manner and it is no wonder oyu are concerned.
If they continue to ignore you you could consider threatening them with a grievance claim, but I personally wouldn't advise that. If it were me I'd write them an email stressing that you have been lookign forward to resuming your career and have found their radio silence quite concerning, but that you are very keen to clarify your points as soon as possible so that you can begin to plan for picking your career up again when you return from ML.
And if they ignore that kind of approach then I'd send them the no holds-barred grievance threat (actually, no, I would raise a grievance, considering that HR themselves are not treating you as well as they should here) I'd complain that you have made numerous attempts to stay in touch and that you feel you have been looked over for promotions simply because you were on maternity leave, and you are concerned that they seem to be ignoring your valid concerns regarding this and regarding their refusal to confirm aspects of your holiday entitlement.
Try not to get too concerned in the meantime - it's not about you. This kind of stuff happens quite often, usually through sheer ignorance (and rarely with any ill-intent). I have yet to meet a mum who had a seamless transition back to work after ML. There is alwasy too much/too little contact, too many/too few work demands and so on. Sometimes the legislation to protect us all actually causes the problems it tries to address. At the heart of it, you are still the professional you once were, you are just readjusting your relationship with the firm. I hope they pull their fingers out and start treating you like a human soon.
A different question is whether or not they CAN treat you like this with regard to promotion and the answer is quite often, yes, depending on the circs. But that's usually by the by, the key thing is trying to get the relationship fixed first, and then worry about the legailities etc if that fails to happen.