As for 'metaphysical object', many contemporary realists still seem to hold disdain in the spirit of the 20th century positivists for metaphysics in general and hold that physics is the ultimate gatekeeper of what is real or unreal, and this is often under the banner of scientific realism.
Metaphysical necessity, possibility, and impossibility, were basically invented by Kripke as an effort to patch rationality in the face of the realization of the radical contingency of our world.
I only try to answer your headline question: When does metaphysics stop being speculation? According to the philosophy of Critical Rationalism, metaphysics stops being speculation exactly at the point when its speculations become testable. Testable means that we can confirm or falsify the metaphysical thesis. Note: To confirm does not mean to prove.
The metaphysical question, uses "nature" in the first meaning. The hope is to explain the second type of "nature". But the metaphysical "nature"... means the "properties and characteristics" type.
From Wikipedia: In metaphysics, Metaphysical solipsism is the variety of idealism which asserts that nothing exists externally to this one mind, and since this mind is the whole of reality then the &
Metaphysical necessity is a question on which philosophers continue to disagree. Our use of language contains a great deal of modal terminology. We say that some things are necessary, some are possible, some propositions are true but only contingently, some things are true but might have been otherwise, etc. Modal talk is ubiquitous and significant and comes in many varieties. Scientists also ...