Trying to answer the actual question

I have looked at the V20 necromancy paths. I suppose there are other paths on the splatbooks, but I don't have the time to make a research.

Generally speaking, necromancy paths seems not oriented to combat. I suppose that's for a reason. A Giovanni necromancer is supposed to be subtler than that and not effective on direct action. With the help of the ghosts, he can be always ahead of his opponents, and prevent the confrontation, or address it always in advantage. I would try to use those resources. Prepared vampire is worth two, or however that idiom translates into English. More action-oriented Giovannis go for Potence.

But you were asking about Necromancy paths effective when you are already in the combat situation. I think you pointed out the better choice: the Bone Path. How to use it outside the morgue? Well, as I said before, the anticipation is your advantage, so if you know you are going to enter combat, you can bring the corpses from the morgue (assuming you have enough dots). Also, in combat, people tend to die, so sooner or later you could have a corpse to take control of. Sure, it's not as direct and deadly as launching flames at your enemies, but let face the fact you will never have anything like that.

It's a shame that Giovanni killed all Cappadocian and Lamias because their paths seems far more useful in combats. If you and your Storyteller wish and can build a convincing story in which your character can learn the lost paths of "The Corpse in the Monster", "The Grave Decay", or "The Four Humours" you would get what you are looking for, or at least the closer you can get with Necromancy. But as a Storyteller I would make it very hard and you would end owing a lot of favours. Be warned!

But you should consider...

I play non-combat characters very often. There's something that have proven essential when your security is threatened: Retainers.

Have a bodyguard or two. They can be more suited to combat than your character, and your character can buy time to do some tricks with them. Ideally they would be ghouls, which supernatural powers would make them deadlier in combat. But if you cannot create ghouls, you can hire regular goons. A Giovanni vampire usually has the resources or contacts to do this; if not, the family can sure help. Dominate 3 would be very useful to avoid breaking the Masquerade, but other means can be used.

This way, you can still focus in doing what your character knows well, necromancy, social work, finances, or whatever.

It has been a long time since I played Vampire, but - not only is it possible to do a group creation followed by individual preludes - it's the best way to do it.

You will probably need to have a gap in time between the creation session and the first Prelude. There will likely be some tailoring needed once you know the characters. But that's not a huge problem most of the time.

Character Creation for the Mechanics and the Meta

The Character Creation session should be used as a chance to have players become familiar with the system if they aren't already, and to discuss a few details of their characters to ensure they will work together and not have too much overlap.

You also have players discuss meta elements. Perhaps two players want to play rivals. They can discuss these details during this process, decide why they're going to be rivals.

Work up the full character, but I'd suggest telling players that no choices are final. Give them a chance to make changes once they better understand the mechanics and the setting.

During this part of the process, you needn't discuss more than the bare minimum of setting details. You know, just enough to make sure the characters will all fit within the premise. "You're all newly embraced kindred in the crazy world of high finance in the 1980s. Think American Psycho meets Vampire Diaries." Just enough to ensure the characters will work in your premise, and no more.

Prelude for the Setting and Story

Once you have the characters set, you can run individual preludes. This is the stage where you begin to introduce setting and story elements, and it can be run as free of mechanics as you and your players are comfortable with.

It need not be less interesting just because the characters are mortals. But making a story compelling does depend a lot on the GM. If you make the journey memorable, it will be awesome.

The last time I played Vampire, my GM did it this way. I built my Assamite, and then we roleplayed his journey from boy, to man, to trained assassin, right up to the scene where his mentor bites him on the neck and then hangs him up on a meathook. It was horrifying, and gut churning, but it was never dull. Along the way, my GM hinted at what it meant for the future.

You mentioned Initiations in Dogs in the Vineyard. As a GM, I've used this idea in preludes for other World of Darkness games (and other games) to help make them memorable. Players make characters, and then I ask them to ask a question we will answer during the prelude. This helps keep them engaged as they search for their answers.

This way will preserve nice balance between player agency and GM mystery. Players have freedom to create the characters they envision within the limits of the premise, but the GM doesn't reveal too many secrets too soon. I've played it, and run it, and it works well.