Well,

this add-in url is

/isc/studio/templates/%25ZEN.Template.AddInWizard.SOAPWizard.cls?Namespace=USER

But when you redirect with Redirect directive, your query part dissapear. And it opens with default namespace %SYS.

To solve it you should use mod_rewrite instead with RewriteCond and RewriteRule directives. I don't give you any examples, just don't have a good place to check it. But I think I can find it.

Usually, the problem which you can face with SSL, is, how to trust self-signed certificate. In your case looks like, you have one server for development, and everybody connects to this one server. And in this case, there are two ways to make this SSL trusted,

  • if you have a domain name which publicly available, from the internet. Internal servers should not be available from outside, just only one server, where you can generate certificate with Let'sEncrypt. And you can make wildcard certificate as well. 
  • you don't have such domain name, and don't have access from outside. So, you should have own certificate server, and make by yourself. I think this way is a bit harder.

For local instances, when you need SSL only on your machine for yourself, you can use the mkcert tool and any subdomain from localtest.me (which goes to localhost) for example.

CACHE.DAT or even IRIS.DAT does not have any relation with operation system. So, you can easily move database files between instances on different OS and versions. But you should remember about data itself, like differences between 8-bit and Unicode. And code, which should be compiled for particular version but also not depends on OS.

You can find some details about database format from my articles.

Sounds wonderful.

But, I hope you don't do it from scratch and reuse some already developed components. Developing editor, from scratch is too difficult, and TDD is the last thing to do on the list. And when you say IDE, it means much more than just Editor. And is VSCode is not IDE but has a lot of possibilities already. You can look at Monaco editor, which can work in a browser, but it is also an engine for VSCode. So, it would be possible to reuse some functionality from VSCode extension.

Every debugging tied very much with an editor of code. So, the first question should be, which editor should be used. The studio already has some debugging options, Atelier, too. I'm working hard now on VisualStudio code, and they also have good ways for debugging and hope I will manage to add good debugger for VSCode as well, but not only me who can do it. George James already has their debugger and introduced their work on the same debugger in VSCode.