Ballpark, how much oil routinely bypasses the filter element in a filter equipped with a relief valve?

My guess would be only one or two percent. The bulk of that amount would be leakage at the bypass valve, which would always occur. On very cold startups, you might see a blip of oil go through the bypass, but I would expect that to subside within a few seconds and not contribute much to the overall situation.

That would depend on the filter manufacturer and their filters. A properly designed and manufactured bypass/relief valve should not bypass any oil unless it reaches it's designed pressure.

Michael, would you have a listing of brands that you would qualify as being of that quality?

It also depends on the oil viscosity and engine temperatures. People continue to use SAE 40 engine oils at freezing temperatures. 40 wt goes through the by-pass valve.

For filters, check out the various studies on line. I've got a lot of cut filters on www.widman.biz
Even one page on them in English

Like any spring loaded valve, those things will leaks when there is ANY flow. As filter media restriction increases(whether its from thick oil or higher rpm flow rate), it leaks(bypasses) more.
Its too cheap and too simple to expect perfection from a filter ADBV.