- There are no special regulations for multi-way bottles (glass and plastic alike). Most stores will accept only those they are incentivized to, i.e. the brands they sell (manufacturers are eager to get them back to be able to reuse them).
- As soon as a store sells a kind of one-way container (plastic bottles, aluminum cans), they have to accept, pay out the mandatory €0.25 deposit for, and properly dispose of each and every one-way container of that kind customers bring in (in household quantities). There is a clearing mechanism, since store's sales might not match their returns.
So, for example, if law allowed only for a small number of different container types (to turn them into a commodity) and then taxed production of those containers heavily, what exactly would happen?
Naively, one would expect companies to cut costs by buying second-hand containers rather than buying expensive new ones.
That would create a market for used containers. For an ordinary person it would be very much like deposit: Buy stuff with the price of container included, then sell the container to get back the money.
It would, obviously, provide a new way to cheat: Produce new containers and sell them as used ones, thus avoiding the tax. But that's just a tax avoidance trick and could be dealt with using the existing anti-tax-avoidance mechanisms.
It would be interesting to know what would be the unintended consequences of such a law.