alacantara i just clean using a microfibre cloth and warm water, don't use anything else on them.
Alot of the guys on here have had success cleaning the seatbelts, easier if you whip them out of the car first, not sure where youd go about getting red ones from!
As above warm water and MF cloth or use raceglaze cleaner. Just don't soak the alcantara.
Another way of cleaning the belts is to pull them right out then shut them in the door so they are out of the car. Then use apc (sown thing like diluted star drops from tesco/asda diluted will do.
Get someone to hold them taught Then jet blast them off.