0 hour contracts are fine - we use a lot of temp labour here.Apologies if rehashing,but what do you want to achieve that is illegal?
Opt out means the willing can choose to work over 48 hours.
You are in the private sector so can choose to appoint whoever fits your requirements. You can't turn someone down because they refuse to opt out - so you just find a different criteria for rejection.
The issue comes with the longer term training of permanent staff. We need a workforce of trained, permanent employees who can flex with customer demand. If everyone is well trained and therefore well remunerated, we are wasteful and not competitive. If we only train the staff who will work more hours when required, then we fall afoul of the WTD.
Your last point works in theory, but the practicality of it doesn't fall that way. We can (and often have) spent years training someone for them to turn around and decide they no longer want to work more than 40 hours. At that point they are virtually useless to us as a skilled employee, so we have to train someone else who can be flexible, and we're back at not being competitive again.