no you cant close the ladder up fully once theyre on , but your only talking about 8 inches or so . 
 yes,you just slacken the wing nut and either slide them up or slide them right off the bottom .  mine came with a diffrent design of the clamp and had a threaded bit  of rod which accomodated every size of ladder .i found i didnt need the rod as long as it was and i just trimmed it down with a hacksaw . the spikes are galvanised steel,quite heavy but after a day i never noticed the extra weight .
 i leave my spikes on always , the only time i took them off was a few weeks ago when i had to take ladder inside a house and the spikes would have messed up the carpet . just slackened the wing nut and slid them off 
 its important Not to fit them too low down ,unless your using them on a lawn or really soft ground .