As for zero days: I doubt carriers can move that quickly without making mistakes. Two months may be a bit long,

I think you misunderstood. This is about "after how many days on the provider's plan" a customer is allowed to switch providers.

In Spain, most carriers sell fully unlocked phones, even when you sign up for a multi-year plan. You can use the phone with any carrier you wish from the start, even though you still have to pay for the original plan, and cancelling it incurs a penalty equal to the prorated amount of the initial discount on the phone price (then they claim inflated "base prices" that are above what one can find at non-carrier shops, but that's a separate issue).

The proposal is at an early stage

Carriers will likely have until well into Trump's administration to make any cha... oh, right. Well, anyway, it could be a thing, "if".