1. What he was supposed to do ? Commit Predator seppuku ?
Actually, yes. In the script and novel he even tries. There are on-set photos of Anderson instructing Whyte on how to hold the dagger. He's supposed to die of his wounds before he can.
Not sure why it ended up on the cutting room floor. It would've at least made Scar more sensible -- he held out as long as he could to stop the Aliens spreading, then died.
3. Well, yeah, that's hard to argue. Maybe she missed it
She do. She gets Scar in the gut, below the sternum where the burster is.
4 These Preds weren't wearing their masks
I appreciate this was Anderson specifically addressing why they miss the PredAlien -- but it doesn't answer why they wouldn't even check
Although considering how quickly the bursters pop in AvP you can almost forgive them for thinking "Well he didn't die until a minute ago, so surely if he was infected he would've given birth by now."