We've been getting new 4* girls lately. But yes, no point in expanding the lowest rarities unless you're Aigis with new classes (and thus, new silvers to collect for CC/AW fodder).

No, speed only affects map movement. Combat is always "your girls attack one by one in order, then bugs attack one by one. Boss gets double turns." Fast forward speeds up the graphics but mechanically it's the same. Mind, map movement can be significant - if any one team reaches the goal, the map ends (all other teams get like 100 turns' worth of free move) with any remaining encounters skipped. This could be bad if you're trying to get the 3* completion bonus. In theory, there could also be complicated maps where team A ought to toggle switch X before team B reaches tile Y but those are easy to intuit as they arise.