Balcony door and window sills - defects in new construction

Since it is a condominium, all defects are handled by the developer.

And that is why all your defect reports go to the site manager, because he is your contractual partner!

Expert should not be an insult.

I did not take it that way either, but just wanted to express two things at the same time: a) that my statements should not be counted twice, but (only) for "window builders"; and b) that I usually experience "experts" as almost the opposite of clueless people.

The installer told me on the phone today (by now two appointments have not been kept) that these are connectable elements,

The elements can generally be connected, but nothing of that can be seen here. The supplier should clarify—by means of a sectional drawing of their connecting profile, for example—how this is supposed to be done here covertly. Such a construction cannot be completely ruled out visually here alone—however, it would be watertight (which apparently is not the case here).

Good evening!
I need your assessment once again. I have now received two proposed solutions:

Solution 1: The first solution, which was proposed by the installer and an Internorm expert, involves sealing the too short coupling by enlarging the coupling joint and filling it with window foam to close the cavities. Additionally, a vapor diffusion-tight sealing compound will be applied on the inside, which will then be covered by a cover strip measuring approximately 20/3mm. On the outside, a suitable sealing compound would also be applied to permanently close the coupling joint.

Solution 2: A 3-part element will be dismantled on site and fitted with a new coupling seal. In this case, the building connection (outer facade, painter-ready reveal inside, floor connection inside, roller box outside, etc.) must be opened and subsequently renewed.

My questions:
1) Can the proposed first solution hold in the long term, or what do you think about it?

2) Can the elements be reused if they are dismantled as in solution 2, or do new elements have to be installed?

3) Does anyone have experience with what happens if the apartment is uninhabitable during defect repair, or who guarantees the security of the inventory (I live on the ground floor in the center of Vienna)? Must the developer cover the costs for replacement accommodation and security service at the apartment?

Thanks in advance
Best regards from Vienna

It won't take that long for you to move out and need a security service, they take care of that in one day

Can the proposed first solution option hold long-term

Out-of-court settlements are not love marriages, but I can imagine it as a feasible way. However, I do not know anyone who could report success or failure in this regard.

they take care of that in one day

In the case of solution 2, I do not want to sing your heartfelt wish at the top of my lungs.