...and learn about other useful shortcuts such as zenGetProp() and zenSetProp().

zen(id) is shorthand for zenPage.getComponentById(id) which as @Vitaliy Serdtsev said gives you access to the Zen component identified by id. document.getElementById(id) gives you access to the element identified by id in HTML/JavaScript document object model. Since a Zen component could comprise several HTML elements, it's usually better to use zen(id).

Security.System is a persistent class with 1 object in it. The ID of the object is "SYSTEM". So this works

set ss = ##class(Security.System).%OpenId("SYSTEM")
write ss.AutheEnabled

The approach above is best if you want to access several properties. But if all you really want is one or two properties of Security.System, you can do this:

write ##class(Security.System).AutheEnabledGetStored("SYSTEM")

It is possible to programmatically add a relationship to two persistent class definitions at runtime, and then compile those classes. That gives you the same result you'd get if you had added the relationship to the class definitions at design time. So I don't think that's what you want.

The term "Relationship" as defined in Caché means "objects of these classes can be linked at runtime, and this relationship will be stored when the objects are saved." So your need to create relationships between persistent objects "as and when they're required" doesn't really match up with this definition. Either a persistent class is in a relationship with another persistent class, or it isn't. It's not possible to have some objects of the class without the relationship definition, and other objects of the class with the relationship definition.

Maybe you just need to substitute one-many relationships for all of your parent-children relationships. One-many relationships are independent; the relationship is not required like it is in parent-children relationships. In v2013.1 and later, you can set the OnDelete action of the one-many relationship to "Cascade" so you get delete behavior similar to parent-children.