It's probably out there in the topic itself but if a genin turns into a chunin, do all the points put into endurance or control automatically raise from +15 to +20?

i.e. a genin has 5 points in control and then turns into a chunnin. Originally the additional hp would be +75. With the chunnin addition it'd become +100- can you add HP retroactively? I'm guessing yes since it'd be unfair to characters who have stubbornly climbed the ranks but I'm not sure.

I was interested in grabbing one or two techniques from this branch of ninjutsu for a character I designed but there doesn't seem to be any low-level techs for it. Does this mean that it's mechanically impossible to gain the techniques written therein? (aka, you can't actually gain two D-rank techniques because... there aren't any.) Or am I missing something?

In response to your first question, yes, when you rank up you gain more HP and Chakra when ranking up.

Now... as for your question about the clay jutsu, Ijutsu and only Ijutsu follow the natural progression law so there's no restriction to using them.