Walkthrough of HoMM3: SoD Campaigns

I also had a similar issue with the hero, but in the mission with Gem. Instead of Gem, I got a random druid, and that's fine, but as a result, no one got the 30 Grand Elves, which made the Walkthrough much more difficult.

Regarding those who have "The Shadow of Death" - Unholy Alliance - Repose as a problem, the issue is that Klansi brings his drinking buddies instead of Gem, and also 30 Great Elves. Seeing this chaos, they all leave for their estates.
I had the same problem, but I think I figured out how to solve it. For this, you need to have a save from the transition from mission 3 to the problematic mission 4 (that window where you choose a bonus before starting the mission). If you're not used to saving in a new slot, I'll attach my save file (https://yadi.sk/d/VO6kjUijW3gkJQ). You need to pay attention to one thing - in the bonus selection window, you can see something strange (or maybe not, I'll explain later) - for some reason, we can switch from the current area, highlighted in red, to the previous area, which is located to the left, where the mission with Helo was, which should be muted and marked with a red stripe. You can then choose a bonus for Helo, and if you click START, his mission will even start. You don't need to do this; instead, click on the area where the "In Search of the Killer" mission was, and then back to the area of the "Repose" mission. Now, if you start the mission, Gem will appear. But there's one thing to clarify - the trick with the unmuted Helo mission doesn't always happen. To make it work, you first need to load the save from the transition from mission 3 to mission 4 and see if the area with the Helo mission is muted. If it is, you need to restart the game and look at the area of mission 3 again. Sometimes it works right away, sometimes you have to restart the game 10 times. Once, the "In Search of the Legionnaire" mission wasn't muted either. I haven't found any pattern here; it just sometimes works. But if we can select two or more missions in this window, Gem will always appear when you click on them.
If you didn't understand anything or it didn't work for you, or you just don't want to bother with it, here's a save file where Gem is already present https://yadi.sk/d/x-v6RQOqDK-63Q. If you don't like the skills that were rolled, don't be afraid to click "replay the scenario"; Gem won't disappear. If you care about the final rating, don't despair - all the previous missions have been completed on 200% difficulty quite quickly and cleanly; I'm probably going for the Archangel rank.
And yes, I really wanted to play this mission specifically with Gem (and of course, 30 Elves are 30 Elves). I found this trick by chance because I was sure that there was a way out of this situation. Since I didn't find any other mention of how to fix this on the internet, I'm writing about it here.