3641144 - is a max length of a string in Caché/IRIS. So maybe the developers just used it to work with arrays in C# as well. I guess it's more a question for them, because I don't have access to the source code to see what's going on "under the hood".

In theory it should work with .NEt Core, it depends on what InterSystems is using from core libraries. When you try it, please tell.

Yes, it is possible to use ECP to access another machine