Hiya, @mgodeck 
Thanks for writing such detailed posts and including the resources you have already referenced!
In addition to the video and github link you shared, have you read this Support Guide? It is generally the first port of call to debug any forms issues.
We also recommend trying to search the forums or look at topics tagged Netlify forms if you haven’t already - it’s likely your question was already asked by someone else! Quickly looking, it seems like this thread Proper way to submit netlify forms with SSR frameworks like svelte / sapper or this thread Form 404 - Static Form in Svelte / Sapper may be related to what you are encountering
If you are still having problems, please provide the following information:
- The URL for your live form as you want visitors to use it
- The URL of your deployed html form. In case you have a javascript form, we need to literally be linked to the html version you’ve deployed, as mentioned above (look for “ pure javascript form”)
- The form name that you’ve set and that shows in our UI
- Any errors or logs from the Netlify build logs, dashboard or browser developer console
- Description of anything you have tried that did or didn’t help or make things better/worse