Good question, tough application.
First important point: the lux levels you will get measuring light in the real world will not match / relate to lux levels specified at a camera. For example, you might be planning to install a camera at a parking lot with .5 lux measured (e.g., Training: Using a Lux Meter) but if you use a camera specified for .5 lux, it will almost certainly be completely blacked out, because camera manufacturers use super aggressive / unrealistic minimum illumination levels (see: Don't Trust Lux Ratings).
Second, a question: Can you not use cameras with integrated IR? There are definitely cases where integrated IR is not appropriate (e.g., wanting color images at night) but, for most uses, integrated IR is an easier and more reliable way to get decent low light images.
For your question 1, what do you mean by low light parameter? What type of low light parameter?
For your question 2, do you mean your specification? As in what should you specify? If so, that's really tough because vendors often lie about their minimum illumination spec, just because Vendor A lists .0001 lux and Vendor B lists .01 lux does not mean Vendor A will perform better (e.g., This Is Why You Cannot Trust Tri-Ed / Speco Super Low Light).
John. This seems like a good undertaking for you to arrive at a spec that can be fair to both the owner and the integrator. You've given us a long list of why this spec (and a number of others, including ppf) should not be relied upon and that's good info; but I would like to see an actual answer to this and similar issues. Rather than specifying a low light number, perhaps a performance procedure would be in order. Say taking a known light source (illuminating the target at specific levels), target and distance (easy to duplicate in an integrator's shop) and then requiring a specific result "category" (for lack of a better term). The Rotakin categories seem to be a good start (A-M). At some point however, only LL cameras or a supplemental IR would be needed. Here is the full test procedure from the UK
As a specifier i could then give, say 10 allowable manufacturers (with other specs), that i believe can meet this standard. And list which categories apply to each camera on my camera schedule. Maybe the manufacturers could eventually say which IPVM Standards they meet or exceed.
I know you don't want to be another UL, but someone should do it... and IPVM is the best. If not you who? Also, what do you think of the Rotakin Test procedure as detailed here.
sounds like a performance spec to me. Drag out the resolution chart criteria.