2. The current logic for material access decides what resources a civ gets access to based on what's available in the immediate vicinity of the starting Mountain Hall, and does not include adamantine currently. I'd expect dynamic resource access to be introduced at or after the introduction of an economy (and derived metal access may become based on access to the source metals as well).
Company warehouses of ye'olde Amazon obtaining materials such as draltha leather (in devlog), or indeed Adamantine from a soon to be doomed deeply dug fortress through trade might power forward other things. Naturally if the guilds bulk buy admantine from company stock, work it and then sell it to very successful & rich mercenaries for a profit margin as personal gear or to town shop vendors/wagon merchants the system will have been working perfectly.
But that's just speculative as we don't super know if a primary-secondary-tertiary chain like that exists, only their supposed broad roles and the guilds haven't nessecarily had a proper devlog to explain, perhaps still forthcoming or it's on the backburner for explicit detail.
Toady, do enemy civs ally together in response to player aggressiveness trying to scoop up settlements forcefully to eventually reach nobility targets like baron, duke and count? it seems like it'd be either too much of a deterrant going that path or a easy way to incur fun if you are the percieved world ending threat of a conquerer.