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.
Your answer can be found in the V20 core rulebook on page 230:
Learning a new ritual can take anywhere from a few nights (Level One ritual) to months or years (Level Five ritual). Some mystics have studied individual rituals for decades, or even centuries.
On the assumption that the final sentence is Storyteller puffery, you can presume that the number of nights is proportional to the dots of the ritual, and that whereas you might pick up a •• or ••• Ritual in a few sessions, a ••••• Ritual might take an entire Chronicle to learn.
Best Answer
I'm relying here on what I consider a common-sense tautology: A rule that says that your traits may not be higher than X prevents your character from having traits higher than X. Extending the premise far enough to suppose a Storyteller would let you buy a trait higher than X under the presumption that temporarily raised Trait Maxima would permit it is, I think, generous enough.
As you posit it:
Now, eventually, the effects of Blood of Potency wear off, lowering the vampire's trait maxima back down to five. It is self-evident that you cannot use dots that you cannot possess, and there is no rule allowing for those dots to somehow go "dormant" until you raise your attribute maxima to a point where you could make them legal again. Therefore, the result would seem to be that your traits are reduced to beneath the new maxima, and the experience points to purchase them lost. ("Sanctity of Merits" doesn't seem to apply here, as Disciplines aren't Merits.)
Alternately, a Storyteller may decide that such dots go dormant, like those of a ghoul who has not been fed with blood for some time (no longer than half a year, then they start to decay). However, given the ease with which Blood of Potency can be invoked, this is an unlikely and probably ineffective workaround.