The best use of Warzone Commendations (for PVP purposes) after reaching Battlemaster (Valor Rank 60)

I just tested this out, and it turns out that if the player with Roaming Mend applied to them takes damage while no other friendly is within 20 meters, it will heal them once and then disappear. 3 stacks of Roaming Mend Charges will remain on the healer who applied it, but they won't do anything even if the healer comes back within range and takes damage themselves.

So essentially your third case.

The exact optimal stats are: 6410 Endurance | 5372 Mastery | 2931 Power 1309 Critical | 774 Alacrity | 701 Accuracy

To achieve this, you want Versatile armorings and hilt (for more Mastery), unlettered Lethal mods (for more Mastery and Power), 2 Initiative enhancements and 5 Accuracy augments (to get almost exactly 110% Accuracy), and the rest split approximately 2:1 between Crit and Alacrity. That means 2 Eviscerating crystals, 3 Crit and 6 Alacrity augments, and 6 Adept and 2 Quick Savant enhancements. Ideally, you want to get Hard Mode Operations tokens (Unassembled pieces) in order to exchange for Weaponmaster gear (which has the Guardian DPS set bonus armorings). Operations drops are also the only way to get the best Mods and Enhancements without crafting.

For the purposes of stat discussions, note that Implants and Earpieces count as Enhancements and are named similarly, with the addition of "MK-1" at the end for the best versions.

Please see this forum post for the mathematics behind these recommendations. The short version is that as a DPS you will always prioritize other stats over Endurance and so choose the low-endurance modifications. Crit is the most valuable stat but quickly enters diminishing returns as you acquire more of it — but Alacrity is also quite valuable and is affected much less by diminishing returns. Power is always less valuable than Crit in 4.0 (given current stat budgets) despite it having no diminishing returns at all, and Mastery (formerly the 4 main stats) continues to be heavily curbed. Oh, and you want that extra 10% Accuracy because it overcomes boss resistances.