The issue you have, Ray, is that you are creating your own problems to some degree. I don't mean that in a disrespectful way, but I'll try to explain..
First off, I concur with regards to Davis support..
The main problem you have, is being on Windows XP. The reason for that? Cumulus MX won't run on XP.. So, you'd need to upgrade a later OS. Unfortunately, you're happy with the older Cumulus - I completely understand your views on that. However, technology has, and still is, moving on.. it is inevitable that people will get left behind unless they keep up with it. In this situation, not upgrading to say, Windows 10 and moving over to Cumulus MX, you are leaving yourself with less and less options, it's impossible for companies to keep supporting older versions of software, eventually their resources have to move forward. Again, please don't think I am prodding you on it, I have no issues with that, it's just for the purpose of this reply.
IF you ever decided, "sod it, I'll do it" You'd get lots of help from folks to get Cumulus MX running, and your website sorted.
Do you have another PC with Windows 10 or something, on it? You could install Weatherlink 6 on it, copy the weather history from your current set up, to a USB stick, and transfer it over to the newer system to do your data fixing and importing?!
When switching over to the WLL last month, I just started my data as a new weather station. I have never used Weatherlink software to gather my data, so there's nothing to import. The 6 years of data I do have, is stored in my SQL data base, and with the Cumulus installation itself. That's the most important thing for me- website comes first, Davis second.
Whenever I do anything drastic, such as do a new website, I leave the current one alone, and work on the new stuff (as and when) on the side, so that it doesn't affect visitors or create any downtime. Same applies to switching hardware or even software. When I am happy that everything is working correctly, I can make the switch nice & quickly, but still have the "older" stuff right there, in case anything goes wrong.
Sometimes, you have to dip your toe in to the water to see how warm it is! It's worth it in the end.