Rent for construction dryer in final invoice/builder - legal?

We assume that the heating device belongs to the company itself, but we cannot say this with 100% certainty.

It doesn't matter. Even if it is their property, using it involves costs. You yourself quoted the passage that heating devices must also be paid for separately. Besides, they don’t install themselves.

We paid about €600 net for the heating device, if I remember correctly. The heat pump was already there, but too weak.

2. Furthermore, the developer installed a hot water circulation system even though we said we did not want it and is now charging 600 EUR net for it. How should we proceed here and can it be removed that easily?

The installation of a circulation system can be absolutely necessary if the pipes would be too long and excessive stagnation would increase the risk of contamination. Otherwise, you should not have to pay for anything you did not order. This also happened to me with the circulation system. It was offered as an option for about 1000€, afterwards I paid 300€ net. I found that fair; after all, I benefit from it and overall the plumber did a very good job. 300€ net for the complete circulation system is quite a bargain. In hindsight, I am even quite glad that I was forced to have it installed.

All costs incurred for the installation, operation, and dry heating during the construction period. This applies to all heating systems, including electric heating systems such as heat pumps and direct electric heating devices.

We understand that we have to pay the pure consumption costs (EUR 418,080). However, the rental cost for the heating device amounts to 700 EUR net. We assume that the heating device belongs to the company itself, but we cannot say this with 100% certainty. Now the question arises whether we also have to pay this rental cost.

All costs...! How else is a company supposed to survive if it cannot include devices in the invoice.

How can/may I offset un-tiled areas (only material or also service?)

Material.