Hiromi Kato is a he. (I wondered about this for 
the longest time myself.)  Yes, he did the 
character designs for Tenshi ni Narumon.  The 
genga and design drawings in the 2 doujinshi are 
the ones that he drew for the show.    To my 
knowledge Kato only did genga.  
Creators/character designers don't usually do 
douga.  I don't have vol 1 in front of me, but 
looking at my copy of volume 2, all the drawings 
in there fall into one of 2 categories:

1) douga/layout drawings/whatever you want to 
call them
2)- character design reference sketches (not sure 
what the official name for these is)  But 
basically sketches that are to be used as 
reference by the animators working on the show 
for how the characters should look in order to 
insure consistency.  These are probably the 
more "detailed" type of drawings you were 
referring to.  As I pointed out in a post a few 
months ago about some drawings of raphael in the 
second doujinshi, some of the poses they are in 
are remarkably similar to ones they are in at 
times in the series and there fore made it to 
cels. But there would have been separate douga 
for these drawn by the animators, but they might 
have been looking at these sketches to do it.

Anyway, hope this answers your questions!

-aethel