There is no limit on backdating planned plots/posts as long as it does not necessitate retconning of events that would have come after - and similarly no limit on forward-dating if it can be reasonably assumed that it'll not conflict with anything that may come up by that point in real time. The spacing of posts as you've described should be fine!

We are also assuming that this is the plot that you'd mentioned before (the rescue underground) - if that's not so, do correct us, simply so that we know if we need account for anything, but even if not, you should be good to go!