Direct link between Ex technician and Ex designer?

Yes. It is the Verification Dossier.
IEC 60079-14:2014 says VD shall be directly understood by Ex competent technicians. VD is the direct responsibility of an Ex competent designer.
So again: the shall speak the same language.

So again: an Ex personnel competency training for a technician shall only be about the glanding… No! NOT AT ALL (not only). It shall be more focused on both theoretical and practical detailed knowledged of explosion protection.
Keep up good work.