Group at 15. Simple as that. Right now the game is basically decided by whichever team gets the first inhib. If you group later than your opponent, it's more than likely that you lose any advantages you made in lane since your enemy will push your Inhib in about 1minute after they grouped.
Concerning the other questions... I dunno really. I tried making sure that each of my player will be guaranteed to get an AP or AD champ (JGL, SUPP, MID = AP / TOP, ADC = AD) and then I spent the points accordingly (while leaving a few points in Tankiness and Utility).
Strong champions... well, all of them have their pros and cons I guess. Make sure that your midlaner will play champs that are strong in mid, your adc plays adc champs etc.. (you can check it in the 2nd tab in the strategy site).
EDIT : Toplane really depends on how do you want to play it
What's mostly common knowledge by now is that you generally set your lanes to "kill" except for Jgl and supp. Supp depends on the pick and your player.
Also, always group at 15.
Strong champs are quite easy to figure out and part of the fun in this game honestly.
Try a ton of different priorizations, should you get beat, either experiment with combinations of champs or just assume you gut beaten by a must-pick and take it away or ban it.
Regarding points: Maybe another time :) But again, figuring out what works and what doesn't is the whole fun of this game.
Unless you enjoy clicking once and watching your team win for eternity ;)