bizarr issue with mp4 Video in eLearning slides. Videos doesn't show and show.

I would also support the move to Vimeo - it made my work a lot easier since I was also having issues with large Captivate files.  It is very easy to embed, and Vimeo allows for a lot more control (for example what screens popup when the video ends, etc.). 

I also found that there are a lot of great ways that Vimeo videos can interact with Captivate through the Vimeo API.  I wrote a blog post if you need to learn that functionality as well. 

https://elearning.adobe.com/2017/12/end-of-video-events-with-vimeo/

You mentioned that a streaming service is too complicated but have you considered uploading your videos to YouTube. Presuming that your learners have access to YouTube, you can list your videos as unlisted, which means that they are not searchable on YouTube's or Google's search engine so they can remain relatively private. You can use a web object to embed these videos and there are a variety of codes that can be added to your embed code to control how the videos look and operate.

Here is a link to a video I did on how you can achieve this.

Adobe Captivate - A Better Embed Code for Video

I have a variety of videos that cover various topics on working with video in Adobe Captivate. Check out my channel when you have more time.

https://www.youtube.com/paulwilsonlearning

While the SWF approach may better protect the video downloads, it will cause issues now with browser's locking down/out the Flash Player. Your users may be able to get around that now by allowing the Flash Player activation but in the next year or so, they won't be able to do that either.

Better to use the MP4 approach to maintain viability of your program overall. Downloading MP4s is just something you'll have to resign yourself to. If an issue, watermark the videos with your brand, or you could look into a streaming service that allows such lock-downs.