1. I modded my game to hopefully make some (I didn't bother with all of them) mega- and semimegabeasts more common. I did so by slightly increasing the number of generated non-mountainous caves, giving them a littersize tag that wasn't too over the top (like 3:5) and increasing their cluster_number (I went with something like 5:7 for dragons and 12:24 for things like giants).
This has worked well in adventure mode in that in most caves it didn't take long to bump into a giant or minotaur, as there were usually several or even in excess of a dozen of them there. I found that littersize, contrary to what I've read somewhere on this forum, might have been an important factor. Creatures which I remembered giving that tag were far more numerous than the others (there were no or very few ogres, despite having the same cluster_number as giants which turned out to be ubiquitous with my modifications). Of course, this might have also been related to biomes, but I digress.
Now, this has so far had next to no influence on my dwarf mode games. Only once, after embarking in a terrifying biome, was I attacked by over a dozen semimegabeasts right off the bat. Other than that, when I embarked on a cave, there were ettins or giants all right, but I have yet to see roaming mega- or semimegabeasts in a non-evil biome. Is that the problem? Am I screwed if I play on something that's neutral or good?
2. Playing in a freezing environment, I did what appeared to be the obvious way of melting my frozen river: I pumped magma into a channel one level below it. The water melted all right, but froze again mere days (of in-game time) afterward, despite there being a 7/7-deep reservoir of magma below it. What did I do wrong?