Trimming hollow doors without circular saw?
I have done this with 100s of hollow doors.   

You need a table saw period for your situation.   The electric planer is the other tool you need - which you have.   But using the planer is not right for that big of a cut.   You will have trouble managing the edges - these doors will splinter on you unless you are perfect.

If you want a half inch gone - 


Remove the bottom piece of the door.   You can usually bump it on one side with  something blunt - that side will push in and the other side should pop out a little.  Pull it out.  If you have serious issue just put a screw in it and tap screw and pull out by screw.   It is the bottom of a door so no one will ever see a screw hole.
Cut door with table saw.  I guess you could use a circular saw but you need to use a guide.   Honestly I wouldn't use a circular saw because these are hollow and you will get kick back and the doors are flimsy.   
Reinsert bottom of the door.
Wood glue the bottom back in.   You will need to put glue on every side.   Clamp it while it dries.  (check 20 mins later and wipe glue)  It needs to dry for a day.
A day later you will probably need to shave about 1/32" off with the planer.   No matter how good you try to get the piece in, it won't be perfect.   The planer will make it look new and will also shave off excess glue.


Also don't touch the top of the door.   Imperfections are really not noticed at the bottom and even if not perfect it will look like wear and tear.   At the top it will be very noticeable and will look like a hack job.   

The other factor of not cutting the top is when you look at a hallway all of the panels should be the same height.   You will notice the distance from top panel to the top door frame trim and if there are differences an OCD person might have a meltdown :)