"Beginning of end step" question
Lone Rider will not transform this turn.

If several abilities go on the stack at the same time (such as at the beginning of end step), their respective controlling players, in turn order, decide the order in which they resolve. 

However, this does not matter for your actual question, because the beginning of end step happens only once each turn. All applicable abilities check whether their conditions are fulfilled at that time, and if they aren't, they won't trigger this turn. At the beginning of the end step, you did not gain the necessary 3 life for Lone Rider to transform, so it won't happen, even if you gain life during the end step because of Soul Warden triggering off the Crested Sunmare horse token.