First, no. There is no good, hard rules on adding negative qualities during game. There have been suggestions and precedence that have been set by Addiction rules. But, for the most part, it's up to the ST. Because of this, I can't really give you a RAW explanation of what to do.

However, I CAN give you better on your second question.

I've had a number of games in my campaigns where I've added negative qualities for things that players have done during the game. A few times, it was addictions. But one of them was an Enemy. During the game, the runners had been sent against a corp executive that had some information for their actual run. While doing the run, one of the players thought it would be a good idea to blackmail him. I had let the player know that this guy was a pretty high-up executive and blackmail wouldn't go over well. However, they continued on with their plan. It had succeeded, since he had a good idea about it. However, the blackmail data had been caught by a rival group that wanted to deface the exec. In short, there was a fire fight and some fun with jumping over moving cars. In the end, one of the group had gotten away with the blackmail data before the runners could give it back to the exec and the exec escaped.

Now, the one that had been doing most of the blackmailing and talking has a corp executive with a lot of resources wanting him dead. What does this mean? No more taking runs from this corp...and sometimes, they would have to deal with a group of runners or corp security as a distraction during other runs.

The player liked the idea and took the negative quality. And, after a while of having runs sometimes go pear-shaped because of a group of newbies wanting to cash in on the bounty on the one runner, he finally paid up on the negative quality and I gave him the chance to eliminate the exec.

In short, negative qualities are something that you can add to players that will give them different hurdles for doing things half-assed or screwing up. And you still give them the ability to use Karma to buy the negative quality away. But I wouldn't constantly do it. Once ever three games, and only when they really deserve it, seems fine. It's not so much that it would make the game more gritty. It just makes it kind of unrealistic to carry around that much negative baggage. If they earn it, they earn it. But I wouldn't slam with it too much. And you shouldn't reward them with Karma or give them more for giving them the negative quality. They earned it from doing whatever got them the quality.

I'm following the spellcasting instructions in page 281 of 5e.

• 1 Choose Spell : Manabolt
• 2 Choose target : Ganger (Willpower 3)
• 3 Choose Force : (limit magic*2 = 10) 6
• 4 Cast Spell: Spellcasting + Magic [Force]

Sorcery is a skill group that includes spellcasting, so this roll becomes 9 + 5 [6]. The interpretation is roll 9+5 = 14 dice and count up all the fives and 6s that show up, these are called hits. If the number of hits larger than 6 (the Force), it is limited to 6.

For example if you rolled: 1 1 1 2 2 3 3 5 5 5 5 6 6 6 = 7 hits.
Maximum number of hits is limited by the force of the spell, so 6 in this case.

Manabolt is a Direct Mana spell, so it is resisted by willpower (or counterspelling) The ganger rolls 3 dice and gets: 3 4 5 = 1 hit

• Determine Effect

Manabolt causes Physical damage The spellcasting hits (6) - the resistance hits (1) = 5 net hits causing 5 boxes of physical damage to the ganger.

• Resist Drain

Manabolt has a drain of Force(6) - 3 = 3 in this case. Shamans use Charisma + Willpower to resist drain. Note that the number of spellcasting hits (6) exceeds your magic rating (5), so this is physical drain. You need to roll 3 hits (fives or sixes) on your Charisma + Willpower number of dice or take physical damage.

• Determine Ongoing Effects

Manabolt is instantaneous, there are no ongoing effects.