Move speed is 100% cap. 50% is the cap for on gear move speed affix.
100% is the cap overall. You can reach 100% with skills, sets and talents or you can reach 100% move speed by 50% MS affix on gear and 50% MS from skills/talents/sets .
If epiphany, I think it can increase MS Bonus to 150% or maybe it keeps the bonus as the normal 100% cap but the on gear move speed cap becomes 62.5%. Asking the real questions here.
Fun fact; Your character has default 100% move speed but if you add the 100% bonus MS (from skills, sets, talents, affixes) , you can get 200% Move speed or move twice as fast.
For momentum, it only counts the bonus I'm pretty sure so 100% bonus means you get the full 50% DMG+ , but it once counted your own move speed as well as the bonus. 50% move speed at momentum (5) means you achieve 25% increase in DMG instead of the full 50%.
Also you can lose the effect of momentum or lose much damage because of the ice slowing Debuff and in arena, the slow trophy Debuff.
0% move speed means no movement; see Sloth seven deadly sins gear without move speed.