Ok, I see your confusion. BUt I have to quote myself from the other thread I linked above.
You tried to change the RTD link from HTTP to HTTPS: obviously it was not working. From that thread:
Quote
2nd, WHOLE RTD links (designer).
Invoking the API or an RTD link with the SSL version is not going to help. This because you might be invoking the designer with an SSL url, true. But everything loaded by it and into it will still be non-ssl, and so mixed contents will still happen.
On top of that, technically, the SLL and non-SSL versions of an URL are technically different domains. This means that creating mixed contents with the two will also cause issues with cookies and sessions (which are set and valid only for one domain).
So, DO NOT ATTEMPT changing that link. It will only be more issues.
Back to your original question: by "standalone" I mean what you are already doing, which means you'll be fine. What will make non SSL page not working is only the eventual inclusion of them into an SSL environment (like the popup/frame I mentioned), but as this is not what you're doing I see nothing to worry about.
You're linking to the RTD which will be a non-secure page, true. But that's the RTD only, not your whole website.
All that said, I just have to repeat my initial one as the final answer for you: I'm sorry, but for now you cannot convert RTD links to https.
But I also have a good news!
For your specific setup, I created and hosted the "bridge" page right in the RTD servers, so I was able to access and modify it.
As you switched to HTTPS also that bridge needed to be edited. I did so, but I also forced the RTD thumbnails (which will show in your cart) to be returned as SSL. As I said, direct link to itmes like images CAN actually be secured.
By doin this change, I at least guarantee your own cart will always appear fully secure, while previously it was not because of the non-secure image from RTD