Can't say I agree with a lot of this.
Yes the UK's mess is (nearly) entirely self-inflicted. That is just not true for the Eurozone.
For one, the idea of an optimal currency area is pretty specious. Would half the continent of North America be an optimal currency zone without a central budget?
The importance of the debt-to-gdp ratio is disputed by MMT, so at least not entirely uncontroversial.
Jumping to a parallel dimension that doesn't include the Euro will of course not magically solve all deep structural problems in the Italian economy. Even if you could actually do that instead of the incredible disruptive Euro withdrawal. But as far as insufficient demand is weighing the economy down, regaining monetary sovereignty would clearly help. The UK could have changed course at any time. Not so if you are under the sway of Frankfurt.