0. For PvP you can get free decks, so cards should not be your concern
1. If you like opening boosters buy booster. Buying cards individually and selling boosters is the more efficient way. You get based on time, and state of your reserves, and daily boost, you can also complete quests and achievements.
2. Play what you like, both should be viable.
3. That mechanic is called "charges", and you need upgrade to apply charges, you can not have more charges, that you have upgrades. But do not worry most cards have more uses on lower charges, that they have in EA's times.
4. Just create a lobby, someone will join, around this time on most maps you should find someone quite quickly..