Teppy

Thanks but sadly this solution won't work either because then it will do it for every user logged in rather than a one time thing at a specific time / occasion regardless of who / how many are online. Thinking I'll have to wait for source for this

To add your own char sprite, you need to put the sprite sheet into the resources/entities folder.
 
To let others download your client, basically zip up the resources and libs folder along with the Intersect Client.exe and files pertaining to it, and then upload that somewhere and give people the download link to it (Short answer)
 
As for the quest system, it is rather common sense as to how it works. Any specific questions?

Pretty sure you'd need to wait for the source for this one

I don't see a viable way for this to work efficiently until we get the source, unfortunately! (You could have a 0 cooldown ability that someone can spam use, but I think they'd have to keep clicking it.)

Sadly this would be pretty tough to do without the source.
 
Only possible thing I could think of is making an event spell, where the spell spawns an NPC named Block Wall or something, and then obviously make it so it can't be moved through. Only issue would be knowing when to automatically despawn/kill it. You could make it have low health and require other players to attack it to "clear" the wall, but the issue with that is people could then run around casting it congesting areas.