This problem has different solutions depending on how you want to play.
If you're just looking for the achievement, one solution to your problem is to do whatever missions you want, choosing the side of good, until you get relatively close to your level point. Then go into a town and steal (for minor karmic reduction) or lay waste to entire towns (for major karmic reduction). You can quickly lower your karma this way. Then, once you're neutral, go kill some random creatures until you level.
(If you're looking to steal, a good place to do it is at Dukov's place… he's got stuff everywhere.)
The "nice" thing about this system is that if you stay on the path of "good" the whole time, you can do this at each achievement point: be bad until you get to the right karmic level, level up, get your achievement, and then restore a save to get back to your good self. You can quickly get "good", "neutral", and "evil" achievements for all levels on one playthrough this way.
(I did this, because I don't really enjoy long-term playing as an evil character… but I do like my achievements!)
You could also do this as an evil or neutral character, but it's much easier to quickly drop from "good" down to your new goal, than to work your way up.
If you find that you're close to leveling, and you're at the right karmic level, and don't want to take a chance on changing your Karma by doing a story-type adventure, you can always go wandering looking for random bad people to take out. Going near the Washington Mall can get you some XP fairly quickly, and you can go inside the capitol building and clear out large areas without picking up any Karma, positive or negative.
Yes, you can do that using the command setstage <quest ID> <stage ID>. In your case, the quest ID is 00014e8b, the current stage is 55 and the desired stage is 60 (The Wikia article you've linked lists the stages and their IDs). The exact command you want is setstage 00014e8b 60.
Best Answer
The game has a way of protecting certain "critical" NPCs that must survive in order to continue the game - they are marked as "Essential."
Most essential NPCs that follow you or lead you will defend themselves if attacked. However, if they sustain damage enough to be killed, they will simply be knocked unconscious, and they will get up as if nothing happened later.
There are some side quests that it is possible to fail if a key NPC dies, but in almost every case where the death of an NPC would prevent you from finishing the game, the required NPCs are marked as essential.
The Fallout Wikia has pages for each character, and there is typically an information box that specifies whether or not the character is essential, and under what circumstances they get or lose this attribute.
In the specific case you mention, your father is marked as essential during the trip to Rivet City, so there is no danger of him dying during this trip. (Do note that there are spoilers on that page if you haven't finished the game...)