Since a regen is most sensitive and selective right at the onset of oscillations your detector is doing its job and its level is good.
Are your red/green LEDs working correctly? They consume 10 mA or so of that 20+
It may be tough to get a good MDS measurement because the Rx input impedance is more than likely not a good match to the sig gen.
The real test of sensitivity is can you her all the way down to the ambient band noise.
Do I understand you correctly that you hear no background hiss or noise but still hear signals?
What are the voltages measured at the pins of U1?
U-310 ought to be fine as a sub for J-310.
If I'm not mistaken the U-310 is good well into the UHF range, since its noise figure has a 450 MHz specification. Its current and threshold voltage specs are in a good range.