Phrase or idiom for funnelling efforts in wrong direction

An option is trying to carry water in a sieve. As this book explains, it means:

to try to achieve a goal in a manner that dooms the action to failure, using a totally inappropriate means for attaining this objective

That is, it means to do something futile. Examples of this phrase (in various wordings) in use are:

It doesn't look like I've accomplished anything today. I might as well have been carrying water in a sieve.

So, you could use this to describe the people. For example, you would say that

the band trying to make themselves ready by arranging their chairs properly are trying to carry water with a sieve

Or,

The girl is trying to swim by reading books. She may as well be carrying water in a sieve.

Another option is barking up the wrong tree, or:

Pursue the wrong thing; to take the wrong approach.

Don't ask me for a pay raise. You're barking up the wrong tree. I have no authority to give anybody a pay raise.

However, this phrase has the connotation of being a bit more aggressive than the previous one. If you are judging the people who are doing the wrong thing, then you can definitely use this phrase. But, if you want to take more of a neutral tone, I would recommend the first phrase.

If the people concerned are fussing over details while ignoring the big picture, you might say they are bikeshedding:

implies technical disputes over minor, marginal issues conducted while more serious ones are being overlooked. The implied image is of people arguing over what color to paint the bicycle shed while the house is not finished.