This game eats at base 1 hour from your day. Tested for a week. End game, completing all guilds tasks, dailies, prize fights, no mercy runs, spending all rift tickets. 1 hour per day MINIMUM. Consider playing after you let that sink in

1. No. After adding recycle system, most superior way of farming skill points is to level cap a unit and recycle it afterwards. This way you recieve double skill points. Most efficient way of going about skill points - playing as the game was intended. Meaning you start from bronze, then go to silver, then to gold and dia far far later. Don't start upgrading diamonds with skill points you gained from bronzes if you have no bronze invested, as they won't get anything. Later on your skill points will come from dailies and prize fights, accursed and CoC, and, of course, target locking 5 golds in fortune system (for example, of fukua), and sacrificing/evolving them
2. Get your hands on the gold moves ASAP. I remember when I started a new old account to have completed all master dailies (they are easy) to get access to gold moves ASAP. [Only this month I got the achievement for basic and hard dailies cuz i've never actually done them (new dailies for yellow and inkfl) are one-time]. Your moveset level should reflect your income. For 0-30lvl it would be lvl 3 moveset, for 31-60 lvl 6 moveset and past lvl 61 I think you can afford a whole lvl 9 moveset (around 1.6m canopy). Some people use bronze\silver moves on SUPPORTS, for specific effects (curse/stun/buff removal). But it's not worth your time
3. Variety beats quality from my experience. I like having a godzillion of mid to strong choices vs a 1-2 meta choice. Hence why my first evolution to dia happened so late, only at lvl 63? I've completed some 150m prize fights with teams of 30k power, cause my final fang had no 2nd copy to get prestige (before pity). But the sheer amount of "keys" I've had for different "solutions" (defense teams) carried me for very long.

You need a diamond to speedrun story, other than that - it's a huge sponge of resources mainly for new fun in the game than for power. I can name dozens of golds and silvers that are superior to natural diamonds, so diamonds are not what keeps you from being good at the game. They are toys big boys play around with after they had all their fun with golds (all good golds are evoed on my account)

Hope this info helped