Which UML 2.5.1 semantic variation points affect required stereotype property enforcement during XMI round-trips?

Context

When exchanging profiled models between UML tools, the handling of required stereotype properties becomes unpredictable. The UML 2.5.1 specification (clause 18.3.1, Annex C) deliberately leaves stereotype application semantics and property constraints as semantic variation points, meaning compliant tools may reject, warn, or silently ignore missing required tagged values. Major implementations diverge: some enforce at save time, others only during explicit validation, and others permit incomplete applications indefinitely.

Constraints

XMI 2.5.1 interchange does not mandate serialization of default or missing required property values, so round-trips between tools with different enforcement policies can silently drop or default tagged values. The fUML subset also lacks standardized Profile runtime interpretation, compounding the issue for executable models. UML 2.6 has not yet resolved these variation points.

Questions

1. Which semantic variation points in Annex C most directly govern required property enforcement during XMI export and import?
2. How can a modeler verify whether a target tool will preserve, default, or discard required tagged values without trial-and-error migration?
3. Does any conformance test suite exist that exercises Profile interchange across vendor implementations?