If you do a blind scan, it doesn't care whether you have any transponders entered for the satellite or not, as it just scans the entire spectrum looking for signal peaks and then checks whether the peaks are transponders it can decode.
I just looked up the manual for your receiver, and it's got me confused too! I see what you're saying about it having a longitude entry field for each satellite -- but if you go by the picture on that page, I believe that that must be the orbital longitude of the SATELLITE. I don't know where you enter YOUR longitude, but there MUST be a way, otherwise USALS mode wouldn't work!
OK, I think I found it! It's just in a stupid place... If you have the satellite motor selection set to USALS, then under Motor Setting there's a place to put your latitude and longitude. You need to have the latitude because of perspective issues -- a degree of orbital longitude near the horizon won't be the same linear distance on the sky as a degree of orbital longitude at the height of the arc.
To answer the question of why USALS mode can be selected independently for each satellite, two things: first, some people say that they need to use DisEqC 1.2 mode to accurately locate satellites at the fringes of the arc for their location. It's never made any difference for me, and frankly, if you have to do that, either your aim is off or the USALS algorithm in your receiver isn't the bona fide USALS algorithm! Second, some people have a combination of motorized dishes and fixed dishes, so for the fixed dishes they'd set the motor option to "off" and select the appropriate switch options instead.
So anyway, now that that's out of the way, here's the way to aim a dish with USALS:
Set your motor latitude as accurately as you can by hand. Set your dish altitude scale to the declination value that your motor manual will tell you how to calculate from your latitude. Turn the motor on the pole to face south as close as you can by hand.
Then, if your motor isn't at its zero position, use the receiver's motor setting menus to tell it to go there. Verify it at the motor. If it's not there, move it there with the motor manual controls, then reboot the receiver.
Now, select the satellite you wish to aim for (it should be near south to make things easier, but doesn't have to be if the rest of the settings on your dish and motor scales are accurate.) The receiver will turn the dish to the appropriate location. Make sure an active transponder is selected. If you've lucked out and aimed the dish dead south before turning on the motor, you'll have signal. Probably, you won't. Go out and turn the motor in azimuth very slowly til you get a signal. Then scan the satellite and see if you get all the transponders you expect. If not, try moving the azimuth very slightly again, and if that still doesn't get you dead on the satellite, try the motor elevation or the dish elevation (very slowly -- best to just lean on it first without loosening any bolts to see if it makes a difference!) Then, go to other satellites and see if they're strong too. If not, again either the motor elevation or dish elevation are slightly off. Repeat until all the satellites you care to receive are located accurately.