I will go ahead and expose my ignorance here. I know that when you read rochuts in tenor clef, you drop a flat. What about reading them in alto? do you drop 2 flats?

Go down a major second for the easiest time transposing. So the key of D becomes the key of C, for example. That would be "adding" two flats each time.

It would be easier to get a treble clef version. Treble clef to tenor, alto, or mezzo soprano is straight forward, and finish by playing treble clef as written. That gives you a way to expand your range up by thirds.

You're transposing up a minor 7th which in terms of keys is the same as down a major second plus up one octave, so it's 2 flats added.

To figure out the change in key signature needed for a transposition, draw a circle of perfect fifths, and find how many fifths you need to go around the circle between the keys, and what direction you're going. For each fifth up you take away a flat or add a sharp, and for each fifth down you add a flat or take out a sharp.

So for example in this case, by changing the clef a written D becomes a C, so that's down a major second. If you go down from D in a circle of fifths, you first reach G, and then another fifth, that's C. Two fifths down, two flats added.