You can absolutely return it within the 14 day window, even unpacked.

During the reflection period, you may open the packaging. You may try or fit the product, just like in a normal store. But you are not allowed to do more than what is needed to judge the product. For instance, you can try pants. But you cannot wear the pants for several days and then return it. If you do this anyway and the product loses value? Then the seller may charge you the damage. The seller needs to inform you of this before you buy the product.

https://consument.acm.nl/aankoop-dienst-annuleren/bedenktijd

I have sent back products within the 14 days, including smartphones that I tried and decided that I don't like it. Some sellers also extend the 14 days to 30 days.

Changing the terms later is a completely valid point though and it should be possible to reject any updated terms IMO.