Alternatively, let's just pay people a living wage and offer them safe working conditions.
Unions are not the problem here, it's the working conditions. Unions form as a reaction to working conditions / terms. If there wasn't a problem, there wouldn't be a union.
Why are societies not allowed to chose certain models how to deal with some economic or employment situations? Why would only capital be allowed to pool but not workers?
Because this ends in a race to the bottom. Employers have much greater power than employees as long as the latter are individualized and only act according to their individual best interest. Society is better when we do things for each other - and sometimes that involves things like not being a scab.
They can and they do - who says otherwise? If a model is very pervasive this also is expressed in people moving to other countries, which in the EU isn't that difficult (and conversely, there are also people moving the other way. A bit like a market for societal models).
So really, individuals do make decisions and act on them on many levels.
They surely can - thanks, in big part, to the unions. Employers are sticky, working conditions cost money. Left to market mechanisms only, a company with good working conditions doesn't use this as a way to pouch workers from a competitor with bad working condition - instead, the company makes its working conditions even worse, or gets outcompeted.
Unions are not the problem here, it's the working conditions. Unions form as a reaction to working conditions / terms. If there wasn't a problem, there wouldn't be a union.