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.

Like many things, it depends. Your calculations are quite reasonable and 372 should give your vampire enough, in fact it has a decent margin of error in it. It has a decent margin of error because a healthy person can give more than 1 BP in a night. I don't have the book handy, but I'm pretty sure they can give up to 3 BP before their life is endangered. Also a healthy person can give blood more often than the medical guidelines indicate. Further, the vampire can, at least to a degree, self regulate and avoid using unnecessary blood. You will still average well above 1BP a night, but a vampire that avoids fights and is deliberately conservative about blood use can probably come in under 2-3 BP a night on average.

So, as you say 372 will provide enough, and even give some safety margin. But that is 372 adults that are at least healthy enough to "contribute" three times a year. You are asking how many would be in a community to give you those 372 healthy adults.

And that depends. You want the birth rate to be equal to or greater than the death rate, but death rates are highly variable in different time periods and different parts of the world depending on circumstances. In the United States, for instances the crude death rate is 8 per 1,000 people per year. Going with that number and ignoring the fact that that is per 1,000 in the entire population and we are only looking at health adults in our 372, he can expect to lose around 3 of his herd each year. That means to be sustainable he needs 3 that are about to become old enough waiting in the wings. If we say they are "adult enough" at 16 and we want this to be indefinite we need 3 children at each age between newborn and 15 waiting around. So, in addition to our 372 adults, we need 48 children waiting around.

Incidentally, the crude birth rate in the US is 14 per 1000 people, so for our 372 adults we could expect an average birth rate (if comparable to US) of 5, easily more than the 3 we need.

Currently, this brings us to 420 to be sustainable. But, you also won't be able to count those with chronic infirmities that prevent them from contributing, like the very elderly. But how many elderly you have also depends. If the death rate is high, you probably won't have many. If you have customs of senicide (killing or suicide of the elders), then again you won't have many. If the vampire has a firm hold and wants to keep the population to a minimum, he might well create such a custom or else "cull" his herd of any not of use to him. If he does this ruthlessly, than 420 is enough with a decent safety margin. But, if he doesn't do this then he will have some additional people. In the US our population over 65 is 13% of total. With a base of 420 people, this makes 483 people total (13% of 483 is 63), so if this is a rough estimate of the number in his population he can't eat than we are up to 483.

I'm ignoring the temporarily sick because (unless there is an epidemic hitting a large percentage of the population) he can easily bypass them for a week and just come back. In short, they are absorbed into the safety margin.

So, I've played rather loose with a lot of the estimates and erred on the side of the vampire being cautious, but a population of 483 could sustain a vampire forever. This of course adds in the assumption that there won't be any huge deviations from average. If there is a war or plague or famine that wipes out a large portion of the population, he will have trouble.