Similar to Nutaku, if you scroll down, you will see an event banner. That banner will say which platinum unit is the drop unit. I don't have the banner image, but you can use this as reference: http://aigis.gcwiki.info/?%C7%FC%BC%D9%A4%CE%CA%F5%B7%F5 . The same thing goes for increased drop rates. There are none atm. Maybe in the next item collection/star event.
You mean Fuuri/Free (Khuri in Nutaku)? She's pretty useful, especially for the current event with all the undead. Her dps is better than most higher rarity VH from what I heard. Never got a chance to use her myself.
Yeah, all rank up bonuses are the same. You will also get a platinum sailor at lv. 50 if I remembered correctly.
None I believe.
Should also mention that DMM's TP also has silver units which are tradable for 10K gold and a specific silver unit. Many people would recommend you get an angel unit asap for certain missions, though it's up to you whether you want the silver angel or wait for the gold angel Eln to appear in the TP again. You may need to wait awhile for Eln since she was in the last rotation.