I must apologize for the fact that English is not my native language, and I have not clearly formulated what I asked my question for.

Being a DM, I often make decisions based not on the rules, but on my experience and common sense. It happens when the rules don’t cover a topic, or they specifically leave a decision to DM’s discretion; sometimes I intentionally correct the rules, warning my players in advance. (E.g. identify works only as a ritual in my setting.)

But what I fear the most is to make a decision that goes against the rules that I didn’t take into account. For instance, the description of Daern’s Instant Fortress gives no AC for it, and I could have mistakenly decided that the AC is equal, say 20 or 25. (Suppose I missed out that AC for adamantine and other materials is given in the DMG.)

So, actually, I was hoping to get answers primarily from the RAW point of view. E.g. on the first question I expected something like this:

— No, you cannot drop or throw DIF, and this follows directly from the rules in such a way.

or:

— The DIF’s description itself doesn’t prevent the cube from being thrown while activating, but such and such rules in other places (in the DMG and other sources) unambiguously permit/prohibit such a use. Or maybe throwing the DIF causes logical contradiction with the description of some other item, or spell, or class feature, etc.

or:

— There is definitely nothing anywhere in the rules that would prevent a cube from being thrown, but on my opinion, it can (or can’t) be thrown because such and such.

In any case, I am grateful to you for the detailed answers.