Hi POOKEY,
1/ I understood the 15 seconds was to ensure that the gear uplock was totally clear of the uplock hook. Otherwise, I thought, the sign would say"Hold out until the green downlock light is illuminated" or something.
2/ The gear handle has to be down to align with the gear logic that warns of "To low....Gear" and such things. Which you will get if you try to land with the gear lever not in the down position.
Cheers