"It seems that BJP has a historical moment that determines today's legal ownership."

What does "logically" mean, and what does "the structure of the argument" mean?

Here logically does not mean "true."

It means:

If someone accepts the premises of the BJP's argument, then its conclusion follows.

That is the logical structure.

For example:

Premise 1:

A temple existed on this site before the mosque.

Premise 2:

Whenever a mosque replaced a temple, today's legal ownership should revert to the temple.

Conclusion:

Therefore this mosque should not remain.

Notice that logic only asks:

Does the conclusion follow from the premises?

It does not ask whether the premises are historically accurate or whether Premise 2 is morally or legally justified.

that is the structure of the argument and I am describing the form of the argument, not endorsing it.