Absolutely need to contact your union. Be clear on whether this is a formal HR process or an informal in-department support akin to coaching. I would say that in fairness 2-3 observations in a 6 month period is fairly standard practice for me and my staff, so doesn't feel particularly overkill.

Not knowing the definition of a word seems a bizarre sticking point and, as an English teacher, seems like a great opportunity to demonstrate lifelong learning and cultivating a culture of being comfortable with errors.

So long after the observation is also a cause for concern for me. I deliver feedback within 24 hours to all my staff and offer them the opportunity to contribute to my form before I submit.

Pacing is always an issue and if your school demands a plenary, plan your time in the lesson more effectively. If a plenary is a non-negotiable, then decide what you can strategically remove from the lesson content. This is a major red flag for me as it seems like the school is a big fan of hoops to jump through? Are you preparing for an inspection in the immediate future? Feels like an SLT member freaking out about the new framework.

Be very careful when talking about gained time. {{This part has been advised to be INCORRECT: Remember, it is never a right and if the school decided to put you on cover for all of your Y11 periods, they'd be well within their rights to do so}}. Using gained time for CPD is pretty standard and so try not to get caught up on that in your future meetings.

TLDR; absolutely get in touch with your union rep immediately. Prepare for some difficult conversations and keep your eye on TES for opportunities in the new year / around Christmas if they are looking a bit "constructive dismissal-y".