A Cainite must spend one Blood Point for each extra action she wishes to undertake in a given turn. Note that you may exceed your normal limit of how many Blood Points you may spend in a turn by doing this.

Okay. The places where the Path charts say "successes" should instead read "Path Levels". So, a character with Movement of the Mind 3 can attempt to lift a 200-pound man, a character with Lure of Flames 4 can create a bonfire, etc.

What do I roll to Cast a Path Power?

Willpower roll, at a difficulty equal to the LEVEL OF THE EFFECT YOU'RE TRYING TO EVOKE, +3.

So, if you're trying to lift 500 pounds telekinetically, you'd (a) need Movement of the Mind 4 even to try it; (b) roll Willpower versus difficulty 7. If you're trying to move only 20 pounds, you'd roll versus difficulty 5. One success is typically needed to cast the magic, though the Storyteller can always decree additional successes are needed for exceptional actions (for example, conjuring a bonfire in the shape of a pentagram, or using telekinesis for delicate work).

A [Perception] + [Alertness] roll is still needed to place a conjured flame at a distance from the caster (use standard firearms difficulties of range, etc.). Oh, and the Weather Control Path's Manipulation + Survival roll should be deleted and replaced with the standard Willpower roll; the "strength of the new environmental force" is governed by the caster's dots in Weather Control, while the lightning bolt always inflicts 10 dice of normal damage.

How much blood can I boil with Cauldron of Blood?

Theoretically, as much as you want. However, boiling more than six Blood Points increases the number of successes needed (and remember, the difficulty is 10 at this point). So, boiling seven BPs requires two success on a Willpower roll (difficulty 10), boiling eight requires three successes, etc.

The rule I've cobbled together is that a vampire can boost a stat to a maximum of twice its normal value (so a 12th-generation vampire with Str 3, Dex 3, Stm 3 could, by spending nine turns and nine Blood Points, raise herself to Str 6, Dex 6, Stm 6).

Are you on crack, Dean? This means that player vampires can raise their Dexterity scores to 10, and elders can pump themselves up to 14, 16, or even higher!

Number One: Mages empower themselves through their Avatars and Quintessence, allowing them to warp reality itself. Werewolves attune themselves to Gaia and the spirit world. For vampires, the ticket to power is the Blood. We're talking about the undead here - the night-fiends of legend and myth. They ought to have a little power.
Number Two:To raise stats to grotesque levels requires massive blood expenditure, which simply shouldn't be an issue in most realistic chronicles. Notice the example above. The 12th-generation vampire's just spent nine of her 11 Blood Points. Assuming she had that much blood in her body to begin with - assuming she'd already replaced the automatic point spent on awakening for the night - assuming that humans are just lying around like Pop-Tarts, placidly waiting to be fed on - she's going to be ravenous at this point. She can't use more than two dice to resist frenzy.
And yes, elders have more Blood Points available. But to refill their Blood Pools completely, Methuselahs and the like must completely drain four or five humans - murdering them in the process. Not the smartest move for beings whose Humanity scores are already teetering on the precipice to damnation.
Just to reiterate something here: If your vampire takes more than five Blood Points from a given victim, that victim is going to need hospitalization. If your vampire takes seven, eight, nine Blood Points from a victim, that victim is probably going to die. There's a reason for those Virtues and that Humanity stat, and this is it. And even the most vile Sabbat warrior can't indiscriminately walk down the street, shredding victims left and right, without suffering consequences.
Number Three: Blood spent activates for "a scene." In this case, that time needs to be measured tightly. Once your vampire spends vitae, the clock is ticking on how long it remains potent. This means that a vampire can't "hulk up" on 10+ Blood Points, immediately recharge herself on whatever derelict the Storyteller happens to place conveniently in her path, and then drive across town to kick her enemy's ass while she's still "supercharged." My common-sense ruling here is: If you have time to stop and replace the blood you just spent, you just wasted your rush.

All that having been said, if my eminently reasoned arguments haven't swayed you, here's an alternate system mandating more controlled blood expenditure. A vampire may raise her stats to the maximum her generation allows (so long as this isn't more than twice the normal stat). This is done per the normal rules governing blood expenditure. Once the "cap" is reached, she may raise the stat further, up to the maximum of twice her normal stat - but once the stat has been raised to the level desired, the extra energy stays with the vampire for a single turn. (This is the "Popeye's spinach" or "Hulk Hogan's miracle comeback" school of blood expenditure, allowing the vampire to perform a single heroic feat.)

Example: A 12th-generation Brujah with a Strength of 4 is in a tough fight. He spends a Blood Point to raise his Strength to 5, the maximum allowable to a vampire of his generation. Feeling that he needs more help, he spends two more turns "adrenalizing" his Strength to 7 - but once the stat reaches 7, he may use the "adrenaline rush" for only one turn, after which the stat decreases to 5 again (although the Brujah may repeat the process with additional Blood Points).

American Kindred:   zuh-MEE-see.
European Kindred:   d'ZHIM-uh-zee, SHAH-muh-say, or d'ZY-muh-zee.

Brujah:

American Kindred:   BROO-zhuh.
European and Spanish-speaking Kindred say:   BROO-hah.

• Spending Blood on Physical Attributes
• Spending Blood on Celerity

Well, yeah. How 'bout this: The roll to hit a target is Stamina + Athletics (difficulty calculated as if the Assamite were using a firearm), and the blood-spitter can expend as much vitae on a shot as her generation's "Blood Points/Turn" rating allows. However, all the vitae must be expended and spewed forth in one turn; otherwise, the caustic blood burns the Assamite's mouth. So, effectively, a 13th-generation Assamite can spend one Blood Point on the power, an 8th-generation Assamite can spend three Blood Points, etc.

Assamites cannot drink the blood of other Kindred. But can a 'normal' vampire drink the blood of an Assamite?

I'm working on this answer.