It's all down to your gut feel
Reading through this excellent discussion, I can't help but feel the missing (intangible) piece of the puzzle is gut feel.
There's an example somewhere of David looking at his lists and then ignoring them and pruning a tree.
For me this is the other side of the coin - having a complete inventory so you know all the things you would like to do (like includes want to, have to, really should do) allows you to goof off and do something else (assuming you're happy to do that - your gut isn't telling you it's wrong).
From the overwhelm perspective - I think it'll come down to gut feel on 'Do I really have time to do Action A now' - if yes, then put on the action list. If no, put on Someday Maybe and review in X months. If in X months it's become 'That would have been nice but I'm not that bothered about it now' - then prune it. If it re-surfaces in the future, then it can go back on the list.
Go with your instinct - and recognise you can't do everything. You can only eat so many cookies before your stomach says no more!! GTD just helps you identify which cookies are the best to eat (Chocolate chip, every time).
And as for having a shovel telling you where to dig - superb, TesTeq - great image