You need to be careful importing a single aspect of OW ruleset.
For instance, parrying multiple melee attacks might seem wonderful, but DW gives characters a lot of options to mitigate the number of attacks not found in OW. Characters in OW do not need the perternatural speed ability to make multiple attacks, therefore moving a character out of engagement range with, say assassin strike, doesn't work with OW rules. In DW, that character just forced the other character to either move and sacrifice all of its additional attacks (even TWF is a full action) or abandon melee combat altogether.
By introducing the 1 parry for all attacks by itself, you are stacking the deck too far into the defender's favor.
Even in OW a character can only take actions on his turn. Therefore when a character uses the overwatch attack he is interrupting the active character (even in the middle of a run action) and temporarily becomes the active character until the overwatch attack is resolved. The defending character is no longer the active character until the attack is resolved. If the defending character is aware of the overwatching character he may dodge if possible.