I actually found someone who did a "Let's play" on YouTube, saw the comment about hard mode, went back through and got the sword then finished the Volcano (some nice puzzles in there, love it).
I've found 4 (actually 6 now) pets (the noise is perfect IMO).
I think everything you have is pretty good for secrets, some are hard, but unless I'm missing a bunch of them, I don't think any are too hard.
I did notice that I got 21/20 first kills on "green slimes" (the ones that look red in the volcano), not sure if that's a bug (the number and/or the name), I didn't go back and check my XP math.
I'm really curious about what looks like portals to me (I've seen 5 of them and noted their locations for next playthrough).
I did not find the green gem the man was speaking of, nor did getting the candle and taking it to the archeologist change anything there yet.
I ended my hard playthrough, no deaths with like 30+ potions, there were a couple times I was down to 3, but I watered the old lady and then as soon as I got the barrel axe I was golden.
This is amazing work bud! I know you just do this in your off time, and I really love it. I'm a developer myself (mostly boring stuff). If you ever need any help / QA / etc, please let me know.
Cheers again!