Sweet and to the point, what's your most dreaded play you have to cover from a mechanics standpoint?
As a wing guy in 5 man mechanics, the worst for me is the 3rd and forever, with trips to my side. I'm keying on the innermost receiver plus the backfield guy. At the snap, I release for the pass almost immediately, and am watching for PI and holding and watching... and watching... and it's been way too long, where's the pass? Oh crap, the QB decided to tuck it and run and I'm 10 yards downfield watching receivers. Now I've got to run back upfield to get the runner and I'm horribly out of position.
What is your nightmare?
Not at all saying anybody is wrong, but, if 3rd/forever is greater than the 10 yards you drifted downfield, what is the angst? Why run back upfield? If you have done a good job of keeping your minimum 6' restricted zone clear, and the 12' area to limit line outside the team area, you should have enough room to avoid action on the field, and immediately OB. In the event that action goes farther OB, you should have time to see it developing and get wider, and/or move farther down the OB area, or even onto the field of play, if necessary, to avoid conflict with the players. If he isn't close to the LTG, the precise spot where he goes OB isn't critical, and you can identify that spot in your mind from many yards away - downfield from the spot - and move to it when the action clears. And, your cross-field partner can get the spot, if you are forced away, and it isn't a close LTG play. Just move to the approximate spot, look across, and get the final spot from him/her.
Now, 3rd and 8 (or similar) is a bit of a different story. You may drift to the LTG and want to hold there, in the event of a close LTG play. (Be cautious about moving farther downfield than the LTG.) You just have to get wide. Just because the restricted area (for coaches) is 6' wide, doesn't mean YOU are restricted to 6'. You can back out as far as you need. Perhaps get prepared for such a play by asking players/coaches to move away from the LTG extended, so you can get there, if needed. Tell them you need to be in good position to rule on the LTG. Outside the team area, you have at least 12' that should be void of anything - no photographers, no ADs, no VIPs, no advertising, nothing (not even ESPN - I had, and won, that argument once). That should be more than enough room to back out as much as you need.
If you've gone downfield, there is no real reason to move quickly back upfield. Heck, that's a great position to see a hold at the point of attack, etc. Just keep your sideline area clean, and you should have plenty of room to do what is needed. Certainly, if you have a clear path, and only a few yards to go to get where you need, fine, move back upfield. But there is nothing wrong with officiating from upfield, with the play coming toward you. The S & F do it all the time.
Even in a crew of 7 (or 8), the H is supposed to move downfield on apparent pass plays, so this is not unique to crews of 5. (The line judge holds on the NZ, even in a crew of 5, at least here in Texas).
Robert