Yes, he was ThunderClan deputy to Bluestar between Redtail and Tigerstar.
According to the book Secrets of the Clans, ThunderClan's original (first) medicine cat was Cloudspots.
Featherwhisker was ThunderClan's medicine cat before Spottedleaf. He was also Spottedleaf's mentor.
I hope you don't mind if I improve this a little Tigerclaw... Redtail (former ThunderClan Deputy) , Oakheart (RiverClan Deputy) , Rosetail (ThunderClan Elder) , Lionheart (ThunderClan Deputy), and Spottedleaf (ThunderClan medicine cat) die. Bluestar (ThunderClan Leader) loses her seventh life. (Hey that's me :)! ) Raggedstar (Former ShadowClan Leader) dies between Bluestar's Prophecy and Into the Wild Thanks -Bluestar
Graypaw's mentor was Lionheart but then after Lionheart dies Graypaw becomes a warrior, so as far as I know he does not receive another mentor. He becomes the mentor of Brackenpaw when he is Graystripe.
By Sunrise, the oldest cat in ThunderClan is Mousefur (I THINK). Hang on, lemme look, yes it is Mousefur, 'cause she's older than Longtail. And they are the only two elders in ThunderClan in Sunrise.
Lionheart
The ThunderClan Medicine cat.
Cinderpelt
From Thunderclan: Spottedleaf&Bluestar Riverclan: Silverstream Shadowclan: Yellowfang Windclan: Onewhisker
Dovepaw(ThunderClan), Lionblaze(ThunderClan), Toadfoot(ShadowClan), Lionheart(ShadowClan), Whitetail(WindClan), Sedgewhisker(WindClan), Rippletail(RiverClan), Petalfur(RiverClan) all travel up river to unclog the river from the beavers.
No, Featherwhisker is medicine cat before Spottedleaf
Dapplepelt is not from ThunderClan. She is a RiverClan warrior in the Warriors book series by Erin Hunter.