Yeah, you would get Rita at 14 UP if you got her from the revival mission. Usually, when it is not 1.5x drop rate, players get the unit from their first revival and focus on the main event. IF they finish the main event, they would try to farm for RCs to get a second copy or wait until the second time the revival pops up to get another copy when they have gathered enough RCs. All it comes down to is you deciding where to spend your SCs. If it was 1.5x drop rate, then farming RCs would be a whole lot easier, especially if it were during a DMM break like last week.
Yes, every event unit has their own revival. I've only been to Marie/Beatrika and Rowanna's revival. Skipped out on the former and only bought one copy of Rowanna since it was 1.5x drop rate week and I had an overabundance of Bonboris waiting to be fused to her. If you're wondering how frequent revival events are, from the five main events I've participated in, only two revivals popped up thus far.
Yeah, units that were introduced before summer 2015 (I think) were placed in the revival summon, which is supposed to end after the current event. I think it runs for about 1-2 months. That means Nanaly, Sybilla, and all those good black units that are in Nutaku now are in this type of premium summon. Unless you're a fast learner and can use the newer classes efficiently, newcomers are usually recommended the revival summon over the current summon because they are more familiar with the units there and there are fewer unique classes.