ESD mat to mains??

[PointyOintment]

Maybe. If the boxes are metal, they should be. If the boxes are plastic, probably not. Check continuity/resistance between the screw and the ground contact that a plug would connect to.

[Richard Crowley]

It would be easy enough to measure the resistance between the round ground pin and the screw head(s).  Some receptacles are made so that the mounting strap is internally connected to the round-pin safety ground.  DO NOT under any conditions poke a meter probe into either of the two slots.  From your own accounts, it does not appear that you are experienced enough to go poking around like that.

BUT:
1) When you can just use a mains plug with only the ground pin connected, what is the point of fooling around with the screws?
2) Even if the screws are connected to the round pin, that doesn't ensure that it is actually grounded.

You seem to be concentrating on secondary issues, and ignoring the primary issue: Are your outlets REALLY grounded?

[aqstephenson]

tested both outlets with outlet specific testers (one gfci and one standard) and both read normal. neither registered an open ground, so they appear properly constructed. of note, however, is that when the gfci outlet tripped, it took out the normal one as well. it would be easier for me to ground to the non gfci, but with this 'series' setup is this an issue?

my mat-to-ground wire ends with that circular attachment shown in 1st pic, and it doesn't appear that there is a way to connect that to the green screw of the 5-15. is the little hole in 2nd photo showing where you were saying to connect a wire? use a screwdriver to bring the plates (gold and silver colored) together to capture the end of the wire, and then connect the other end of the wire to my mat wire's circular end? if so, whats the best way to connect a wire end to that circular mat-to-ground end? or would you recommend stripping the mat-to-ground wire and connecting directly to the 5-15? might be easiest, i'd prefer not to do that unless ultimately it was safest. very much appreciate your patience and input!

[Richard Crowley]

Safest and most straightforward is to cut off the ring-lug crimped terminal, strip the wire and terminate it to the 5-15 plug as it was designed.  The plug is constructed like that to greatly reduce the possibility of any stray strands of wire shorting from one pin to another.  If at some future point, you need the ring terminal back, it is trivial to crimp another one on the end of the wire.

[karoru]

tested both outlets with outlet specific testers (one gfci and one standard) and both read normal. neither registered an open ground, so they appear properly constructed. of note, however, is that when the gfci outlet tripped, it took out the normal one as well. it would be easier for me to ground to the non gfci, but with this 'series' setup is this an issue?

Why did the GFCI outlet tripped? If it was because you or your tester shorted live and earth then it's absolutely okay - modern circuit breakers are not selective (it's very hard to do equivalent of old "slow-blow" fuse with predictable outcome).

Go to your nearest hardware store and buy a mains plug. Connect only earth terminal with your ESD mat grounding wire, plug it into whatever socket you got at your house and you're okay. It's safe unless you can't discriminate between a connector that goes to the "mouth" of "sad face" outlet and others, in which case you got bigger problem than ESD safety:)