i'm not the creator of the addon, but i can answer a few question;
1) Can we change the antlions to other npc like Starship Troopers SNPC addon?
maybe this can work? edit a map and replace the antilion by the snpc but since they're not using the same ai i'm not sure that hammer entities will work with them


4) Are mission objectives possible like in the 2005 game where you have to find the crash site on outpost map.
if you're really good at mapping, yes it's possible to make a map like that, i started making a objective map one year ago but i never finished it

Thank you for the reply. I managed to extract outpost map to have a idea about how it has been created. I managed to replace the entities with Starship Troopers SNPCs + Troopers NPC + SST Weapons. It works almost fine except that no bug fly over the outpost high fence so they don't move after spawn. However, I changed some path_corners to make them move in front of the fence but they can't attack it of course.

You can have a view at this: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2061243737