My pvp spec (at 85) is sub but I went down into the combat tree for
Improved Sprint. Maybe move improved sprint down to the first tier of the combat tree. This might be something to look at for your mobility problem. Now sub gets a 2nd chance to get on target with shadowstep and combat has a 2nd chance to get on target with imp sprint...but more risk due to sprints longer cooldown.
I like the idea of freedom for palis but what is your view on shaman's Purge? Really...at 19 purge serves little to no purpose other than making paladins freedom cooldown useless. Also consider purge on Mages mana shields or other much needed survivability cooldowns such as Void sacrifice for warlocks. Remember it's 19 and utility/survivability is limited...at 85 you have options when you get purged...at 19 you get purged and you get destroyed...I don't think there is a good case to be made for keeping purge at 19 twink or indeed the leveling brackets.
While some burst in the bracket is clearly OP I don't consider Ambush among those.
There are few parallels you can draw between 19 endgame and 85 endgame pvp but there is one important theme that both currently have...put simply: You can beat the heals. At 19 less burst means more "Do or die." Situations. You return the flag or you protect your flag carrier...or perhaps you allow more people to fight in a single WSG battle which would allow for more wiggle room in that regard.
I mostly play pugs and it's already hard enough to get a decent alliance flag return attempt going. You talk about nerfing burst too much and you will have caused more harm than good...i dunno it's a fine balance.