Dave, to your narrow question, what you've done with the nuts is not wrong but I've found it better to omit the nut on the inside of the rim and put both - or just one - on the outside. You don't want to snug that one/those ones down too tightly, they need room to let the valve stem "cock" a little if need be, rather than ripping out of the tube.
To the broader issue, it does seem like you have a slow leak. Probably slow enough not to produce visible bubbles? I'd leave the tube out, inflate it to a known pressure, and let it sit a few days, then check it. I'm betting it'll be down a few pounds or more...
"Economy" tubes are anything but. I swear by Michelin (don't run their tires, although they're high quality, but the tubes are the best). I'd rather reuse a Michelin than put in a new tube of inferior make.
Frustrating stuff, I know - BTDT. Best of luck.
EDIT - just looked at your pic better. On yours, that rounded internal "washer" stays in and BOTH NUTS GO OUTSIDE (there are two to act as lock-nuts, although there's not much lost by using just one - especially if it helps you balance the tire more easily...), no ifs, ands, or buts. However, I don't see that as causing your leak - but it is better practice to install properly - that rounded section of the "washer" is supposed to conform to the internal curves of the hole in the rim the valve goes through.
