"Le Chaud Lapin" wrote

How difficult is the oral part?. Time is limited so obviously they
cannot ask every thing. Is it possible for a student to slip by on the
oral portion and just do well on practical and still pass?

Download and review the PTS - in general you can be asked about anything in
it. You can fail on the oral alone and never get to the flying portion of
the test. If you do that badly I suspect the examiner will be in touch with
the CFI who gave you the endorsement to take the practical.

Also note that the CFI who endorses you for the practical must also certify
that they have given you instruction in the areas you were found to be
deficient in when you took your knowledge test.

Also, can FAA examiners act as instructors simultaneously or is there
a rule forbidding it?

Depends on what you mean by instruction. If you fail to perform any
maneuver to the PTS standards you are probably not going to get instruction
on how to meet them and then also be rated as satisfactory. Same goes for
the oral.

CFIs who send too many poorly prepared students to examiners will probably
draw the attention of the local FSDO office at some point.

BDS