I honestly can't think of a good explanation for why Vegeta was so weak despite getting a Senzu. It doesn't seem fitting of Vegeta for him to lose Shouki or some bullshit because of Black suddenly getting a power boost either, since his character has always been about searching out strong...
In the anime they can. In the manga it seems like they need to be in SSJG/Blue form to do it, although that's not much of an excuse to justify the "Galu can't tell how strong Vegeta is in base" bullshit when Galu noted Vegeta lost no power from repeatedly turning Blue against Black in their next...
I don't think so. Kame Sennin seems to be a term Galu uses primarily (Kame-Sennin jii-chan) while others like Tenshinhan and Kuririn seem to use Muten-Roshi-sama more.
They said Vegeta was healed enough to get a Zenkai if he weren't maxed out though. And all instances of Zenkais in the original manga only occurred after full healing (like when Galu only got the 33x boost after his healing was complete post-Ginyu).
They said Trunks needed to heal someone on Blue tier even when he was healing base Galu/Vegeta though. Not to mention Kibito had to restore Gohan to full power rather than just restore his base Ki.
Base Vegetto was strongly implied to be above Galu/Vegeta's blue forms though. And this was more or less confirmed with Gogeta (who should logically be below Vegetto manga-wise) displaying (C)SSJB tier feats in the Broly movie.
It seems plot dependent. Vegeta with the Senzu should logically have more stamina than Galu who'd been fighting in base, SSJ and SSJG for a while, yet that Blue Kamehameha against Hit was noted to be a Blue burst of full power while Vegeta seemingly lost power immediately after getting the Senzu...
The 100% probably refers to the full power of (non-mastered) Blue that you can access the instant you transform, since Galu couldn't repeatedly transform to Blue without Ki loss in U6 and Kaioshit said that Galu used Blue as a burst against Hit.
There doesn't seem to be one post-Rosat seeing as Beerus praised the explosive power of Blue when Vegeta attacked him right after transforming. This also doesn't seem to be after Blue drained him either, since when Beerus whacked him with his tail straight afterward and knocked him into a lake...
Never read much more than a volume or two of it since I never had any interest in GX, 5Dicks or anything after the original series. I read the original YGO manga (from Yugi first becoming friends with Jonouchi up to the end of the Millennium World saga) and the YGO-R spinoff, which are honestly...