Spot/partial metering while using TTL flash?
with Canon specifically, when you have your metering set to spot and an eTTL flash, will the ambient exposure be completely ignored in Av mode?
No, it won't be ignored. Av mode is your "fill flash" mode. It measures ambient (using whatever metering you select) and sets the shutter speed accordingly. It then uses the flash as fill. If you want the flash as the main lighting, you'll need to select another mode — typically M — and dial in a significant underexposure of ambient.
Be aware that in relatively low-light situations, using Evaluative metering on the camera along with E-TTL II flash will cause the camera to reduce the ambient exposure to compensate for the additional light that the flash will be expected to add. This is not true of other metering modes (such as Spot); in those cases it's the photographer's job to determine whether to reduce ambient exposure, flash exposure, or both, and by how much. (FYI, this feature has been informally given the name NEVEC, and you can see your camera's exposure settings change as you switch the flash on and off.)
There are a number of other complexities involved in E-TTL II flash photography. If you're not prepared to get into them, I recommend that you stay with Evaluative metering for both camera and flash, and use Av for fill flash and M (shutter speed set to X-Sync, which I think is 1/250 on the 70D) for "close to pure flash." I believe there's also a Custom Function you can set which will force Av to use X-Sync shutter speed, thereby making it unnecessary to switch to M mode for "close to pure flash" but removing your ability to use fill flash.
The open-source LightZone Project: http://lightzoneproject.org/
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