Acceptance of condominium "conditionally"? Conduct a visual inspection?

Since nerves are frayed, I hope things have calmed down a bit.

From the abundance and nature of the questions, I get the impression that you are not sufficiently prepared/informed.
The handover is probably still some time away; but when it is time, things will get serious and the BT will definitely not be friendlier at that appointment.
You can make the fight against your BT a life purpose. For that, you need time, sufficiently deep pockets, and a combative nature. But it’s really not fun at all.

Regarding the situation:
Where should the light in the apartment come from in the evening? Lights and lamps were not included in the sale. So why agree to such an appointment (proposal)?

I already wrote: Handover and acceptance are not the same. Moving in and inspecting is therefore not harmful as long as acceptance has not taken place. When the acceptance is supposed to happen... we don’t know.
But: The BT will presumably not allow you to move in unless acceptance is also declared at handover.

In short:
Acceptance “under reservation” (reservation of what? That there will be light?) is not possible.
Specific defects must be specifically complained about. If this is disputed, the differing assessments must be noted; if necessary, send a one-sided protocol afterward to the BT as proof, though that is the worst solution.
Pay attention to § 640 II of the Building Code during acceptance.
The BT has the house right until handover, so can exclude third parties. But this is short-sighted on the BT’s part: You can get around this at acceptance because it is a so-called legal transaction declaration, in which representation is possible, see earlier posts. Granting power of attorney for experts is advisable. And of course you may accompany experts (if, of course, the opposite is not provided for....).

Acceptance in daylight!

Not letting the owners into the apartment is a “show of force,” but extremely annoying and therefore effective.
Then he won’t get the last payment, etc. But none of this is fun. The contract situation is also important – and it is not known here.
And you also have to survive 5 years of warranty phase with the BT afterward – but after acceptance with changed burden of proof for defects.

If necessary, spend 500 EUR and get advice from a (specialist) lawyer for condominium law.
Or bumble through with dangerous half-knowledge from the internet.
Or close your eyes and just move in, see above post from nordanney.

Anyway: Good luck!