As someone who has nominated in the last few elections, I would suggest there is no need for a buffer.
Having a buffer time would make it fair
If you squint really hard then it can appear unfair - but a lot of things appear that way if you look hard enough. Personally I was commuting to work and was then straight into a 3 hour meeting when the current phase (and the questionnaire) went live, but whatever.
I don't think there would be any real advantage to having some lead time - the amount of text in the answers is enough to turn anyone's brain to mush, which means that only the truly dedicated are going to read most of the answers and all the rest will be people checking out a specific candidate that they're curious about.
I also think that over the last couple of elections there has been enough of a gap between the leaders and the rest that the questionnaire has zero impact on the final result. Your answers could lose you a few spots, but they're not going to propel you to the top - as at this moment Martijn hasn't posted his answers but that's had no effect on his popularity at all.
It would also make it fair for candidates in a distinct time zone
I don't think this is much of a problem either. Every candidate knows the exact time in UTC that the next phase starts so they can alter their schedule if they need to. Usually there is also one or two town hall chats - these are guaranteed to be at an odd time for at least a few candidates, but once again you can schedule it. Somewhere in the process most if not all of the candidates will be "disadvantaged" by a timezone difference. If a candidate truly wants to be a moderator they will make it work.
Flipping it around, I think that there's no real way to fix it so that no-one is disadvantaged at any stage, so best to mix it up so everyone is equally disadvantaged at some stage.