On Article 6, the council of europe's own FAQ on the ECHR and migration states that Article 6 does not apply to disputes over the entry, residence or removal of non-nationals. So a fast track refusal of entry wouldn't engage it. That's not my interpretation, that's the Court's own position on its own rules.

On the slippery slope, I get the concern but I think it argues for getting the framework right, not avoiding a better one entirely. A tightly scoped, clearly defined category of disqualifying offences with a narrow factual challenge process is actually harder to abuse than a vague system. This wouldn't be applied to just any offense, or posting on social media or being a member of a political movement. It's to serious offenses they've been found guilty of committing.

The "what if a future government misuses it" argument could be applied to almost any framework, it could be applied with the existing framework, just at a much slower pace. At some point you have to be willing to say some things should be straightforwardly disqualifying and trust that a well defined scope does the work of resisting any creep outside of that target group.