Doesn't meet your detailed requirements, but may meet your intent:
Serfas Thunderbolt. It's not too bright (especially in the lower setting), is USB rechargeable, costs $30-40, but it doesn't bolt onto the bike.
It attaches with elastic straps, so you can take it off and put it on in a second or so. It's small enough to fit in your pocket or messenger bag or whatever. I got one of these on a whim and now I have three; they're my go-to tail light now.
I was a little iffy on the straps, and after years of use I finally broke one. I went to the Serfas site and found they sell replacements for $2.50 each. I decided to buy 4 so now I have extras.
I believe their function is warranted for life too. I know of people who've taken them back to the local bike store chain here who replaced them on the spot, no receipt, no questions asked.