I'm a bit late to this party. I also had a hard time with the lead during the verse, but then came up with a way that seems to make sense to me. There a number of ways to approach this. I'm just sharing another point of view.
Attached is a table that lists the A-mixolydian notes and the chords in Deal.
In the table, the three notes that make up the chords A, F#, D all
map into the A-mixo scale. This is represented by the black boxes. When playing these, any
note in the mixolydian scale because two criteria are met:a) the root note A is in the chord
b) all notes in the chord fall in the mixolydian scale.
What about the Ebdim, C# and B chords?
1) Let's look at Ebdim first. It has four notes, two which map into the A mixolydian scale. Of the two criteria mentioned above, only a) is met. So, the question is whether A mixolydian can still be used. The answer is yes, but since both criteria listed above are not met, it can only be used selectively. I think just about all notes in the A mixo can be used except B. The B note doesn't work because there is no B note in the Ebdim chord. On the other hand there isn't a D in Ebdim chord yet a D note works. This is where some theory and self-liberties come in to play here. The Ebdim can also be viewed as a D7add9. This chord is a derivate of a D chord, therefore playing the D note in A mixo during the Ebdim chord works. Right or wrong, this what I tell myself to convince myself why this works.
2) C# chord. This chord has three notes, only one of which maps into A mixo scale. Neither criteria a) nor b) are met. Therefore A mixo doesn't work. When it comes to this chord on the lead, I switch over to C# mixo and then switch immediately back over to A mixo on the next chord.
3) B chord. This chord has three notes, two of which map into A mixo scale. Again, neither criteria a) nor b) are met. Therefore A-mixo doesn't work. Here, I choose to use the B-mixolydian scale only during this chord. The reason for B-mixo (and not B major) is that it has an A note in its scale. Because of this, and since the root of the song is A, the Bmixo scale keeps the lead grounded to the root. Although, the B be-bop, which is a combination of the major and mixo scales, works well here too since it includes the A and allows for a nice chromatic climb.
In summary, for the Deal lead, try this:
- If both criteria a) & b) are met, then any note in the A mixo scale works.
- If a) is met while b) is not, then A mixo works when used selectively (i.e. avoid the B note).
- If a) is not met, then it's just easier to switch the key of the scale for the duration of that chord.
For Deal, A mixo on everything but the B chord. Use A mixo selectively on the Ebdim (avoid the B note). For the B chord, try B bebop scale. This should add another dimension to the lead as opposed to sticking in one mode.