That's on DMM, Nutaku does zero development on core game mechanics. Possibly not even when it comes to events and such, though we can't really tell what happens behind the scenes.For my two pennies, I agree that Water is easier to start with. In general Water SRs are higher quality, and same with their SSRs should you randomly luck one out.
UE = Union Event. These events are great for players in decent Unions, but absolutely terrible for players who are in weak Unions or outside Unions entirely. I'm not sure if a new player can expect to find their way into a Union that can feed you all the rewards for free... if you can find one, you'll gain a HUGE boost to your Rudra account though.
"Rudra" revival was most likely referring to Rahab revival, which is a Water Advent event. It's probably too soon for you to clear Ultimate Advents at that point though, so it won't be a massive boost to you, but absolutely much better than nothing.