I work a white collar union job in the US where there is no seniority hierarchy. I assume in most professional type union environments this is the case aside from experience requirements for senior job titles (and, to be fair, these would tend to be more strict than in the nonunion world; if the job spec says 10 years experience, you need 10 years).
To chime in on number 2, again my personal experience is the opposite: incompetence needs to be documented and proven to an arbitrator in the worst case. You can't be "incompetent" because of stack ranking or your boss doesn't like you.
On 3, I can only laugh. Union leadership and my shop steward have no say in my day to day work whatsoever other than as a backstop if management isn't following the contract. Even in this case the expectation is typically do whatever the boss says and grieve it later. Exceptions for safety or serious ethical concerns and that's about it.
To chime in on number 2, again my personal experience is the opposite: incompetence needs to be documented and proven to an arbitrator in the worst case. You can't be "incompetent" because of stack ranking or your boss doesn't like you.
On 3, I can only laugh. Union leadership and my shop steward have no say in my day to day work whatsoever other than as a backstop if management isn't following the contract. Even in this case the expectation is typically do whatever the boss says and grieve it later. Exceptions for safety or serious ethical concerns and that's about it.