Leah4 this all sounds complicated and I think for that reason alone you need to get it all in writing, just so you and anyone else relevant is sure what is going on.
It does sound from what you are saying that proving that you should have had more of an increase might be difficult. One thing that is clear is that if cost of living increases were applied to everyone's salary in October, yours should be as well. I appreciate that you were not actually receiving any money at that point, but according to the recent links I posted about the Alabaster thingy, any increase that takes place during maternity leave (not just when you are receiving SMP) means that your SMP should be recalculated on the basis of that salary.
I would be astonished if your boss knew that, so I think she is genuinely thinking backdating to December is the correct thing to do as that's when you returned from maternity leave and started earning more money. However it is your entitlement to have your SMP recalculated on the basis of your new salary and any adjustment paid to you.
Having said that, you may decide not to pursue that. As I say, your boss won't be aware of this requirement, (which actually gives a positive advantage to women on maternity leave.) So her failure to do this will almost certainly be through ignorance rather than any deliberate attempt to diddle you out of your entitlement.
The reason I say you may decide not to pursue that aspect is because SMP recalculated on the basis of a 2% increase won't be very much, and you may decide that if you want to stay there and maintain good relations, it might be politic not to make a fuss about that. Up to you, but sometimes it's worth considering not pursuing your legal entitlement if, on balance, the overall cost of it in non-monetary terms means you would have been better off not to.
I think one of the problems is the lack of formal structure about your salary system at work. Leaving aside your colleagues for a moment, it is not uncommon if someone is paid above the market rate for their job, for whatever reason, to freeze their salary for a certain period, which could easily include a freeze on 'cost of living' increases. So doing that as such isn't necessarily a problem. Where there may be a problem is if you feel that some of your colleagues are in the same position as you and are still receiving increases. But you can understand that a system whereby there is a clear salary structure, people are doing more or less the same job and the woman coming back from maternity leave doesn't receive the same increase as everyone else is a lot clearer cut that your situation.
She does sound a bit flaky and not quite sure what she wants to do about salaries generally. I do think for that reason you need to get the plan for the next year with your salary put in writing, for clarity. I think she needs to explain properly why, if performance related pay began in 2007, it does not apply to you, what's different about your job that means performance related pay is not possible or appropriate. As I say, if there is a definite decision that you are slightly overpaid for what your job now is, there's nothing wrong in freezing your salary for a time, but she is not clear about this.
I think she sounds as though you have put her on the defensive, so she is clutching at reasons why your salary has not been increased, not being clear about what her thinking is. It might be worth asking for a chat with her, and approaching her in a 'nice' way, saying that you understand her concerns about your salary being over the market rate, but you are feeling a bit confused about the situation, you would like to understand what her thinking is, you would like for everyone's benefit the situation for, say, the next year, to be clarified in writing.
I feel I have been waffling on a bit, so I hope some of that makes sense anyway!