I never know about the chattering - we get it fairly often but I kind of figure if they go back to sleep it doesn't matter!
Yes I would stick to the set times whatever, +/- 15 mins or so. The idea is that you set their clock to expect sleep at those times, and they have two opportunities to regulate ie nap length and morning WU time. I know it's so hard to stop thinking A times but I really have found by sticking to the times (with the odd exception e.g. When she had a 25 min nap at nursery and that was it for the day!) that things have improved for us hugely.
For example, J had EBT of 6.15pm after short nursery nap, and didn't wake until 7.45am today. She's gone down really easily for nap at 12 (normal is 11.45) which is a tiny A time if you think about it. But I expect her to do a short nap on that (though sometimes she surprises me) and she will have her normal BT of 6.30pm - which although the A time may be long after a short nap, the whole day isn't long (11h tops) so we rarely get OT wakings.
Sounds complicated but isn't!