Longer lines will be at least far easier to waterstart...
I am a kind of advocate but ... Free of charge !!! Lol. Just that I like what Pulsion brought me in term of ride extensions in lower range of wind. Now other free ride kite like frs or Falcon or Sky country Spirit v2 ( I put a lot of hopes in my new Spirit v2 12m) are quite similar but still pulsion remains IMO the Queen of marginal wind
About rear line attachment: attaching the line further from the kite is exactly the same as pulling more on your trim strap...
So put the knot in the "middle" and relocate it at the extremity of the rope only if you feel your trim has not enough stroke to depower the kite .
I did not answer on coating : personally i do nothing until kite has to 20-40 hours of ride, then I put nikwax on the cloth. This, in theory, protect from UV, and is repellant to ease the relaunch. Hard to guaranty the efficiency though, but i do it as a safeguard and i feel it has improved the relaunch from water. This can make the cloth a bit less porus in case of wear of this thin 27gr cloth; there is a dedicated post on the recoating .