Okay this is apparently one of those things where you’ll get different answers depending on who you ask (even different Wikipedia articles give different answers), but this is a matter of semantics. No matter what you call it, mixing on a molecular level will result in the release or absorption of (in the case of gases a very small amount of) heat.
Yeah solutions can have any phase of solute and any phase of solvent. The most common example of a solution of gases is the air, so yeah.
No. Gas in gas can not be a solution. The solvent must be a solid or liquid.
Okay this is apparently one of those things where you’ll get different answers depending on who you ask (even different Wikipedia articles give different answers), but this is a matter of semantics. No matter what you call it, mixing on a molecular level will result in the release or absorption of (in the case of gases a very small amount of) heat.