Generally, intensity values like that are unitless unless you specifically work out some scale for them. A fair number of archviz renderers do so in order for them to work nicely with measured IES light profiles. There isn't a *formal* standard with games/the DCC packages used to create their assets, though, as physically-based rendering is just starting to catch on these days.
Incidentally, you very much want to establish some sort of PBR framework so the values you feed into the shader(s) are used in a meaningful context, which should hopefully make sense when you think about it.
Re: quadratic attenuation-- that should again make some intuitive sense considering real-world light follows the inverse square law. You likely will see better results moving over to a simple area light model, though, as point lights are physically impossible. This would also give you a more sensible attenuation model 'for free.'
Lastly, tone mapping is pretty much entirely an artistic process, you fiddle with it until it subjectively 'looks good,' and that's that.