Momentum doesn’t count the base movement speed. only the extra speed from Sets, Skills, Affixes, Myth Stones, Mythic Affixes, etc… I did a lot of Searching on the subject as a Noob after reading lots of Posts about how awesome Momentum is. I had the same problem with the math you do when I first used it.
also, Evasion Nature is Dodge, not Movement. or are you referring to Dexterity?
if you have Dexterity 40 for +20 Movement, 2x +15% Legend Movement for +30% = +50%. that gives +25% damage with Momentum (5).
I’ve used Draught Mythic with Momentum for a damage boost against bosses (+50% Movement & Attack Speed for 5 seconds, 5 second cool down). this takes the place of your Mana Potion.
one of my early Wizard Builds had Movement +50% with Momentum (5), and Draught, and since I was using Comets at the time, +25% damage from Momentum, +50% with Draught activated, with boosted APS of +50%, so DPS was really good.
Execute Damage should be multiplicative with Demonic & Electrocution, and the same with that new Set, Mercenary, in spite of it’s restrictions. I think I did one Execute Damage with Demonic Set for a little while to test it out. the test was worth it. especially best to use that combo with Crushing Flames.
btw, are you using Cool Down to speed up your Chakrams with Whirling Blades? with CD +60%, you can cast WB every 2 seconds instead of every 5 seconds. tested and really cool DPS (not sure it shows well on Stat Page, but WB 5x in ten seconds instead of 2x in 10 seconds…). this will also mean stacking up Shock DoT faster so you don’t have to worry about how much Shock debuff you have at any given time.
if your Rogue is Shock Element, then let your Hireling do the CB & CF attacks. definitely better for using Electrocution, especially when you get enemies HP to 25% or less.