1. HS->SS provides more overall enmity than just SS, because of the combo damage. The way all +Enmity moves (except Flash) work is that they have a multiplier on the damage they do. Since SS does a good deal more damage in a combo, the multiplier is much more effective then. As long as you're using Overpower first, that should give you enough of a cushion to use HS->SS without losing threat. If you find that it's not, just Overpower (and/or Flash if TP is an issue) more before you start going single target.

2. Skull Sunder in a combo > Tomahawk > Skull Sunder by itself. This is outdated by now, but it's still a decent rough guide to how these things work in general. Overpower, Steel Cyclone, Flash, and Circle of Scorn have all gotten significant enmity boosts since that was updated, and other moves have had small tweaks here and there as well. I'm not aware of a fully up to date chart.

3. Yup. Keep an eye on the enemy list, and cycle through in between GCDs. Be aware that with a gamepad, switching sub-targets is not enough to update the enmity gauges in the party list. You have to fully switch targets (hit X). The gauges blink like they're switching to show your sub-target, but they're actually not.

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1. /marking attack1 through /marking attack5 (or /mk or /enemysign; all the same command). If you just use "attack" without a number, it will use the next available number. So you can use /mk attack again and again to assign the next number. I'm not sure what makes that reset back to 1, though. Personally, especially when I play with a gamepad, I only mark #1 anyway, and switch it to the next mob I'm going to focus when the current one is almost dead.