Is it a offence for the keeper to catch the ball outside the 18 yard box if his feet remain inside the 18 yard box?

Answer 1

Yes.

The offense is looked at by the location of the hand-to-ball contact. In this case, the contact occurs outside of the penalty area, so the goalkeeper is therefore handling the ball outside of the penalty area. The 'keeper must be cautioned and shown the yellow card. Play is restarted with a direct free kick from the spot just outside the penalty area where the handling occurred (it cannot be a penalty kick because if the handling happened inside the box, it would not be an infraction).

In some cases, the referee may decide that the infraction was "trifling", which means that the offense occurred but did not impact the game in any meaningful way. This can happen when the kick was not a shot-on-goal and there are no attackers in the vicinity of the infraction who might have been able to play the ball if the 'keeper had just stepped back a little. If this is the case, the referee should allow play to continue and must caution the 'keeper at the next stoppage of play.

Also, there is a possibility that the 'keeper tried to catch it as you described, but didn't secure it and lost control. If this happens, and the attacking team immediately realizes a promising goalscoring opportunity, the referee may play "advantage", allow play to continue, and see if the opportunity develops. Within the first few seconds, the referee may decide that the advantage wasn't realized and call the original handling. In either case, again, the referee must caution the 'keeper either immediately or at the next stoppage of play.

Answer 2

Only the position of the ball matters.