If it is hitting other flipper, then obviously we need to move plunger guide rail to the left.
Is the upper left "guard" post you speak of the metal mini post between the outer left orbit and the inner left orbit?
If so, then getting more left-trajectory would avoid that as well.
It looks like you may have a bit more leftward adjustment available in that posted screw slot.
My plunger guide rail looks to be screwed in about as far left as the slot will allow.
My plunge goes into the outer left orbit, not the inner left orbit FYI.
Gate position should not matter, but it shouldn't change the ball's path (I assume gate is loosely hanging and does not impede speed much either?). Another thing you can do is take a slow-mo video using cell phone to watch for any unfavorable rattling in the plunger lane before it exits. Plunger tip centered? Hitting the gate problematically on the way out?
Setting playfield angle can also affect your trajectory as sixpakmopar says, so it is a good idea to set playfield angle first the way you like it - as well as ensure it is level left-to-right. While you are at it, clean and wax the playfield as ball speed will also affect how it enters the orbit. Shooter spring stiffness will also affect ball speed and orbit entry, so ensure you like your plunger spring before proceeding. (I'm not sure if this is a new project or an existing game of yours).
If you need more "left" than the slot would allow, you could as a last resort: mount rail further left using new hole in playfield (*gasp*) through upper, small hole in rail mount. Or (*double gasp*) bend last inch or so of rail as needed.
How about a picture of your full playfield so we can see 3rd flipper and the orbit entrance - just in case something odd is present there.