The secret of the Illuminati and why they are identifiable yet unstoppable.

[kılıç_mehmet]

Here I would really want to see some proof, that someone would be forced to make such a choice.

It's not even a hypothetical choice. It's a choice of profit. I believe that the most indifferent and money grubbing of all people would be able to rise up enough in the ranks of international masonry to contribute something of value to the interests of masons, while others generally fulfill the roles of middle men.
They wouldn't be forced to make such a choice, they'd make this choice knowingly and willingly.
This is why they are dangerous.

Membership fees, donations, investments, wills and revenue, if I were to make a guess

If there is a membership fee, there surely must be membership benefits.
I wonder what those are.
Besides, investments...Belong to the masonic organisation as a whole, I guess. Wills..Well I'm not sure who would leave their entire posessions to the masonic order, but I could say that this organisation resembles verily the various islamic sects that operate in Turkey, most of whom, were also closed by our founding fathers, many of those who had ancestors who were members of these sects, including famous ones such as the Mawlawi, origin of whirling dervishes.
Those too once had a lots money under their disposal before their closure and and seizing of their assets. The masonic order also presented itself as a religious order in Turkey, and it's still known as a religious order of another religion of no one is really sure what they believe in.

That depends on the situation. Again, have there been such instances, that someone have been forced to make that choice?

Well, yes, as far as I'm concerned, especially in my own country.
Other than that, I'm speaking of a hypothetical, extreme circumstances.
But it doesn't have to be a state of war to make such a choice. If masons would not favor their members over their individual national ties, what purpose would their fraternity serve anyways?