No
No, counter-strike only uses the same engine.
No.
No. Half-Life: Source runs on the Source engine. The original Half-Life doesn't. You cannot use mods made for the original Half-Life on Half-Life: Source because they run on different engines and it won't work.
In Steam, go to Store and search the specific mod you want, or search Half-Life 2. Then at the top of the page where it says All Items, change that to mods and you can browse different Half-Life 2 mods.
No
Yes. If you have a Source game like Half-Life 2, HL2 Deathmatch, Counter-Strike Source, Left 4 Dead, Portal, Team Fortress 2, etc. you can play Half-Life 2 mods.
Yes you do. Counterstrike: Condition Zero, however, is not a mod of Half Life, and therefore, does not need Half Life to acquire.
Go to start menu, type in %appdata%, if there is ".minecraft" when you search, then you can. Anyways, why using demo?
Mods can be downloaded from Minecraft Forums. (See related link below.)
You can still add mods using a different launcher through Demo/Single player you just can't join multiplayer servers. You have to purchase a premium account for that.
yes as soon as you've downloaded the mods you can play the mods while offline.
You NEED to have downloaded minecraft to use mods. There are hundreds of mod installation tutorials for all of the most popular mods on YouTube!