simple before you ever unpause the game after reaching the embark, type "reveal hell" and then go to the top left corner all the way down to hell (lol, this is where all the create-units spawn at... I've been trying to find away to move that point...) if when you get to the bottom level of the map, using (k) to look at the square, the game tells you that there is anything there, eerie glowing pit, slade wall, etc, then it has map data, and create-unit is going to operate just fine... but. if it says nothing.... then the bottom z has no map data on that square and it will have issues. I went through the chances of this, basically you would have to have a surface change of over 50z+ between one region square and the next region square for a 1z+ change in hell; and that's only a slight potential shift of the hell floor on that 1 "region square" being off by 1z from the others... so of your playing on a cliff region with the high end on the top left it increases the chance... Well that means that without actually trying to make it happen, chances are around 10% that you would select a map that is 3x3 and that the top left corner is 50z+ higher than any other region square.
And that's being generous... I've ran hundreds of test generations (every time I make a serious raw change, I run tests for each race effected) and I don't usually even use the find feature (I just take the first random spot they drop me at, and 50 drink/50 meat), basically I'm not even trying to find a sustainable embark. But I've been checking this location and I've only seen ~10 that had a dead tile, out of probably ~200 test runs in the last month. And even if it is a dead tile, its not a total loss.... about 1 in 4 create-unit uses, the game tries to set a flag on the tile the unit appears on, and since it doesn't exist, the game decides to crash. So even with a "bad tile" embark location, your perfectly capable of accomplish the task with a reasonable setup (I wouldn't suggest converting 15 elephants at one time with such a location).
on the sprites: I'm not really a "graphic" kind of guy, I've looked into the "tiles" and other "graphic" issues, and I just don't understand it... we've had a few try to jump on it... I know that dokie made a graphic update... I've not got around to checking it or trying it in the 1.24c environment, but I was told it fixed most of the sprite issues that people were complaining about, all the sprite sheets, and twbt tile swaps etc, I would try that out, before I tried to make a new one. you might have to go back 2 or 3 pages to find it... I think it was in january when it was produced.
Another note: I hate to say it but, I'm enforcing a hit on PAUSE on this issue... Its made my head hurt, and I've wasted a good 16 hours trying to bash it out... I'm going to hold onto it, but I'm going to switch to some other projects for a bit (2 or 3 days at least) I have a lot to get done, and I'm not making progress on it, I think I just need to give it breathing room to unravel on its own.... in my head of course. I'll send you a direct message on the forums if I come up with a solution. or maybe boltgun will figure it out... lol...