For strain-relieve, I've consider it but:
* this will make the PCB bigger however that's not so much of a problem;
* the solder sides will be at the back, whereas front side, the pads act like touch points;
* the extender is for hobby use (repair/calibration), probably only during week-ends, not intensive professional use.
Regarding the 7 connectors, they're standard 2.54mm (.1in) Molex connectors. Readily available at Mouser and Farnell.
There are cheaper alternatives at Tayda electronics or even aliexpress.
If you don't plan to do debugging without mainframes, then none of the extra circuitry is needed.
I'll write a small README along with a proposed BOM.
As for the power good (PWR) high TTL output, it is intended only for TM500x modules which expect regulated 8VDC.
The markings on PCB specify the regulated TM500x voltages.
TM500 supplies filtered but regulated voltages, tolerance of +7.6V (fully loaded) up to +16V!
It is not a problem as these older modules don't use PWR. But it draws around 2-3mA.
In fact, some TM500x modules (AA5001 for example) don't even check PWR when they should.