From this perspective, I don't know how far Reimu is away from Satori's crotch and what she is doing there.

やっと届いたので結界組で一通り遊んできました。しかし、地霊殿なのに温泉絵が本当に少ないですね･･･。よし、ちょっとこいしと背中流しっこしてくる
Is the artist's comment. I don't know any Japanese. Does this say something about the situation?

"Because it reached at last, you played in a general way with joining boundary group. But, although it is the Chirei lord, the hot spring pictures are few truly, don't you think? is,…. Even if, just a little it is dense and with the backs style pee it does "

That's what the artist sayed...I used the translator so I hope you'll understand.

Yeah, I used babelfish too, but is it really.
"pee it does "?
Is Satori peeing onto Reimu/Reimu drinking it??
I looked in a dictionary for the "pee" kanji and can't find them in the original text.

Correct translation is, "It finally arrived today, so I played it as the Barrier team. Despite the name and story, there were very few pictures of hot springs scenes at all... Alright! I'm going to go wash Koishi's back (and have her wash mine)!".

Well, mostly correct. I'm not sure on what nagashikko is usually translated to in english.