If the slatted frame is screwed on at the bottom, the mattress may be a maximum of 14.9 cm high according to the manufacturer. You can find a technical drawing under this link. https://ibb.co/6RTW5Yj
Unfortunately, we have no experience of our own. If no one responds, we recommend that you create a discussion on this topic. Experience shows that more customers respond there than to a product question.
Normally, yes. If your bed base is exactly 140 x 200 cm. The slot is designed for a mattress that is 140 x 200 cm, so it should (without guarantee) be ok with your box spring.
You can choose this option after you put the product in the cart (services). There is a surcharge for the service, but the assembly of the bed is included as well. :)
The slats can be mounted (screwed) at two different heights. In the lower version, the mattress is only about 15 cm above the floor, so that even a pocket spring mattress should not protrude too high.