Thanks for so expanded response! And sorry for replying that late (full time job takes much time huh).
Yes, I'm playing version 1.0, and monsters spawned by firemace spots seems to be legitible explanation, that's weird mechanics xd.
I'm playing default difficulty = 3, and yeah, that's too easy (most likely due to my Doom experience), so I hardly have to use any items or stronger weapons (and – surprisingly – crossbow seems to be most satysfying and fun to use, though it's one of weaker weapons; feels kinda like shotgun from Doom), and since hoarding things is strong instinct inside me when I don't know what I'll meet in following levels I finish most of levels fully stacked :D.
After finishing episode 3 (I just finished E3M9) I'll probably play whole game on difficulty = 4.
Hm, I checked on Doomwiki, and it says that in e2m9 there are 98 monsters on difficulty = 3 (that's so much less than 121), and 117 on difficulty = 4... it seems that amount of monsters was reduced in Heretic: SotSR in comparison to v1.0. Maybe I'll do some investigation in some time :D (e.g. comparising both WADs in UDB).