Having the user fill out an attestation using the Docebo tests is a great way to work around the HTML marking the course as completed. 

Another thing you can do, is if the external training provides the user with a certificate of completion, you can use an assignment and ask the person upload their certificate. (this does involve an instructor or administrator going in an evaluating the assignment though).

Another option is using a SCORM package as the last object and have that serve as the attestation that the user has completed the external course. 

Found this you may find useful. It’s a SCORM wrapper you use to house your external link and set parameters any which the course is marked as  completed

I agree with the above advise with one caveat: If you have multiple scorable LOs that you’re averaging (instead of taking the last one), you could run into a scoring problem using the “test”, SCORM, or xAPI methods. In that case you might need to use an eval/poll question (which has its own limitations), or create your SCORM or xAPI module so that only completion (no score) is reported back. 

This is the approach I take, it works quite nicely. Make sure to include step by step instructions on how to do so for users on all the platforms that are relevant to you.

What’s don’t you like about the observation checklist experience? This is what we use. So far, no complaints, but we do have the trainers fill them out and approve them not the learners.