Hi Phil. On paper a shuffle and swing look kinda similar as they both involve triplets - dividing the beat into three. So a bar of 4/4 is counted as four sets of three - 123, 223, 323, 423 or 1 and a, 2 and a, 3 and a, 4 and a.
The shuffle rythm is the stereotypical Status Quo grrove - dun -der, dun der, etc played by hitting notes on, as you say the downbeat and the third bit of the triplet so..
ONE (and) A, TWO (and) A, THREE (and) A, FOUR (and) A
Often you'll hear it with the downbeats quite short and the "A" quite long which gives that du-duuur, du-duuur feel
With swing, the idea is basically the same except the notes are played for their full value, so with "one-and-a",you would make the "one" last until the "a" rather than cutting it short.
Regarding the metronome, if it's clicking quarter notes then it doesn't really suggest any feel/groove just a pulse. If your playing a quarter note scale, what will make it sound different is how you are subdividing the beats in your head. Weird as it may sound, counting "one and a", etc will make the quarter notes "feel" different to counting 16th notes ( one e and a).
And now, the 2 & 4 idea. The idea of this is to give you the backbeat ( the hi hat in jazz, or the snare in rock, blues, etc) and leave the responsibility of the downbeat to you. I find it really helpful for swing and straight grooves. The trickiest bit is getting to hear the metronome click as 2 & 4 not 1 & 3. Easiest way is to start your metronome at about 50bpm, and you'll no doubt hear it is as the downbeat..
Click two Click four and so on.
Count along like this until you feel comfortable and then drop in a bar of three before returning to four. The click will now be on two and four...
Click two Click four, Click two Click, one Click two Click and so on
Hope that helps.
Cheers,
Alun