Phalanx 2 is the best place for platinum spirits. You don't strictly need 2 witches for phalanx 2 - I've always used the silver witch and gold princess myself, although they need a fairly high level and a bit of support.

You need only one hour of gameplay on most days to play close to optimally, as long as you can play at two different times (about twelve hours apart so you don't waste stamina).

As for events, you could probably start immediately, although many people will advise against it as it's inefficient. It's a matter of whether you prefer getting more units in total to advancing faster (where advancing means you can do better in later events). You could probably clear the weaker maps (which also get you a few SC), so you could get any collection unit without cost reductions, the secondary reward unit from star events, or a copy or two of farm event units.

Regarding the trading post units, Spica (the second best archer in the game) is hands down the best choice right now. Saki is the best ninja in the game and can also be very useful at times, but ninjas are a situational/niche class. She's not nearly as game-breaking as most black units - at least not before skill awakening, which we don't have yet. Also note that you need 100 DC for each awakening, so you may want to save some for that at some point. I'm not sure Cyrus is worth even 70 DC as you may get him or someone better from the shrine in the future, making his purchase a waste.