It highly likely if the chain keeps coming off that something has taken a knock or is slightly out of alignment.
when i was making my d.i.y reel i had this problem for ages, chain too long, not enough tension, too much tension, if i had it slightly out it would come off and do my head in all day.
now ive got it sorted it doesnt come off, but even if it does theres no chain guard to mess with and it only cost £50 not £600...
there will be tension as you pull the hose off, thats normal, as far as im aware all leccy reels do that and all use the same £25 scooter motors.