1) You put cutText command in the description.ext. If you are defining your own GUI classes to put into a cutRsc/titleRsc, then they need to go in that file, but if you want to use the simple cutText/titleText commands, or run any commands at the start of a mission, then just put them in your init.sqs/init.sqf file. Generally, though, I'd use a hint to give instructions for a demo mission, since you have more room to give a more detailed description and it stays visible a bit longer (I think).
2) I'm sure you could have a single script, instead of 3, with a parameter to determine scale of effect and damage. For example, each script does exactly the same thing using slightly different code, which is only going to get more different as you improve the scripts. This would make the script much more maintainable for you, reduce testing time and make it less likely that any single script gets new bugs over time.
3) Please warn people that the EFP videos you link to are combat examples with real people getting blown up, rather than just firing-range weapon-effect videos, which is what I assumed. Most people won't be bothered by that, but I am (no problem at all with having the links; just people need to be able to make an informed decision before they follow them).
4) I think you should not have a height parameter. If the mission-maker wants a higher device, then he can balance an object on a section of wall or something. Seems odd to require an object and then have the fire come from somewhere else. Alternatively, just require a position array rather than an object if you want an "abstract" device. That way, the mission-maker can use an object (getPos), marker (getMarkerPos) or calculate the position themselves.