There are a lot more factors than just skill and ability.Salary should be relative to skill and ability. Nothing more, nothing less.
For example, amount of effort applied for each work hour, number of hours, results, availability, like-ability with employees and customers, loyalty, retention issues, ....
For a lot of jobs what you get paid is also going to be based on your ability to negotiate with whomever is hiring you when you start, which goes to ability and like-ability but also negotiation experience which are not normally useful for every job, so is another skill set entirely.
No there's not. Everything you suggested applies to ability.
Nonsense. One can have all the ability in the world but with no motivation, no character, that person can sit on his ability and not use it. That person can even spend most of his time subverting the company. Ability isn't worth a copper penny if it's not applied.