Yes, these abilities are not rerolls, so you can force the reroll. Which of course may force a rage quit!
Wow-thanks for all the responses. Im experiencing a bit of a "rage quit" argument with a tzeenetch player via text message currently. He is adamantly opposed to me being able to reroll his DD claiming the the roll happened before the game. As for substituting after I force the reroll, they can't do that because they have to pick before it's rolled, so, I suppose what they can do is substitute a die instead of rolling, then I force them to reroll that die, so they substitute a different die. Starseer essentially forces them to use two DD. Either way, if the tzeenetch player is your friend and you bring starseer, you should probably bring them cookies
Depends on when the DD roll occurs. Starseer says "Each insight can be used to re-roll any single dice before your next hero phase." So if the roll occurs before you have a hero phase (as would be if he got first turn/roll happens before the game officially has a hero phase) you would not be able to force the re-roll. One bright side for the tzeench guy is that he has a chance to choose correctly and gain more re-rolls. Just tell him to be a glass half full of blood type chaos player and just know that he may be able to alter every single die thrown in one of his turns (ok not really, but a ridiculous number). Cookies wouldn't hurt either.
I agree, you can't make someone 'retroactively' reroll. I think that substituting is a type of roll (like changing a roll). The current action is that you are picking your roll instead of rolling it, thus resetting the time which you rolled it.