Can't change product quantity in to a dropdown menu and strange button beheavor.

Good evening.

I'm encountering some issues in my product site customization:

1. I would like to change the "product quantity" field in to a dropdown menu and limit it to max 5 Items. It seems I can't find any helpful solution in the forum nor in the Squarespace wiki.

2. I have featured similar products just below the  selected item and if I hover over the "+" Button the shape changes in to a black square. I can't remove or even find the property settings for it, the "website style" → "Buttons" changes doesn't affect it at all. Does anyone know where I can change the appearance?