The B747 (like all other aircraft) slide pack does have to drop about 30 cm before the slide will inflate, so if the catering truck is by the door the slide pack should just drop on to the truck.
On the B747 and most other aircraft (not B737) the door should automatically revert to manual or disarmed if the door is opened from the outside and the slide pack will not deploy.
The only reason the slide would deploy and inflate is if the door is opened in automatic / armed from the inside and the slide pack drops more than 30cm or so.
As for ice on the girt bar I have been flying for nearly 20 years and the only aircraft I've seen this happen is the Airbus A300-600 R and that was fixed by engineering using ice resistant grease around the girt bar. The door would not be able to be put back into manual / disarm initially but tended to defrost by the time the passengers had disembarked. The door would always be guarded and catering informed not to use the door.
In 14 years on the B747 I have never seen the girt bar ice up.
Hope this makes you feel more secure