1 - I like to consider myself an expert eBay-er (my ebay account can legally buy alcohol in the US, and I am myself only 36 years old) but I have not found any ZED-F9P for less than $90 ish. I am aware of the Quactel (sp) modules, and am considering a set for some RTK experiments. Is that what you're referring to?

2 - I've thought about replacing with ocxo but the pads are tiny. My soldering skills aren't nearly as good as my ebaying skills.

3 - it is outputting 16 Hz, chrony is doing dpoll -4 poll 4.

4 - I have done tempcomp (it is not active for the post referenced by this HN post). Fun stuff and probably my next writeup.