"The respective chords are shown in boxes adding up to 12."

Yes, that's a basic 12 bar (or measure) blues chord progression.

"Each box represents on bar of four beats is this correct?"

Yes.  It is assumed to be in 4/4 time signature, meaning 4 beats per bar/measure.

"Are these chords played as dominant seventh chords A7 D7 and E7"

That paper does not specify.  You could play them as dominant 7th chords, or just basic major chords.  But you could also play it as 5th & 6th diads in an 1/8th note rhythm as indicated in the tab on the sheet above the boxes. 

So, you have a few options here.  I cover the basics of the 12 bar blues form & how you could play simple versions of them in this tutorial.

https://www.guitartricks.com/tutorial/539/

I cover how you can play the progression as diads in this tutorial.

https://www.guitartricks.com/tutorial/218/

"chords each for four downstrums."

Again, that sheet does not specify how & when to strum.  So you could do that.  But you could also do the diads written on the sheet above the boxes.  Again all this is covered in my tutorials linked above.

"Does the above diagram illustrate these seventh chords."

No, it does not.  It indicates to play an A5 diad for 2 1/8th notes, then an A6 diad for 2 1/8th notes, then again an A5 diad for 2 1/8th notes, then again an A6 diad for 2 1/8th notes.  The tab does not indicate if they 1/8th notes are to be played straight or in triplet swing rhythm.  Fortunately I also cover that in the tutorial linked above.

"How can I use guitar tricks jam tracks tool to practice this to"

Go to this lesson & play along with the video or the Jam Track.

https://www.guitartricks.com/lesson/11558

"what speed should I set my metronome to"

The sheet does not specify.  But the jam track on my lesson page is 80 BPM (Beats Per Minute).

Hope that helps!