I don't know for sure, but is your AI alerted? I had issues with that when I created mods for Crysis. The AIGotoSpeedStance (or any similar AI node for that matter) won't work if the AI is alerted. That's probably why they added the Force parameter to that node for C2. Anyway, if that is the problem, you can try to first use a AI:IgnoreState on each Ai and setting them to ignore, but that means they won't attack or even notice any enemy/player. Or you can keep using the AI:Execute, which isn't handled the same way (I believe a signal is sent to AI to request an action on top of its current behavior, rather than trying to take control of the AI and fail). Replace those second & 3rd nodes on each line with an EntityId and an AI:Execute and see if it doesn't work better.
Also, to assign a new archetype to an entity (who knows, as you said maybe that'll solve the problem, although I don't remember any parameter that could cause/fix this issue), just open the Database View, select the library then the archetype you want (under the Entity tab), in the viewport select the entity that you want to change, and press that little icon at the top that looks the same as applying a material, it'll apply the selected archetype to that entity. You may have to reload its scripts for it to work right away though, you'll find that button in the roll-up bar above the Create/List/Remove Flowgraph buttons I believe.
Good luck with that, the AI in Crysis can be quite clever, but it's also very tricky to control and have it do interesting stuff!