Re: Shield Mastery
Channeling Scourge isn't terribly powerful unless you are putting in a lot of effort- then it should be decent as it fits the cost.
If you get positive energy, you can do a burst of 1d6 * 1/2 level to undead only a couple times a day. And since the save DC is based off of Charisma (not your strong stat), they will likely make a saving throw. So at 10th level you are looking at something like 17 damage (9 if they save) against undead that are surrounding you for your standard action. Not terribly overpowering.
If you get negative energy, you are blowing at least one more feat so your party doesn't hate you. Selective Channeling to do 9-17 damage to living things still isn't impressive. Now Channel Smite might be an interesting option. With the right judgements up, a +9-17 points of extra damage on a couple of key attacks per day isn't bad. But still, we are looking at a level delay in Inquisitor, and a two feat investment.
As for Shield Master, the feat is written poorly. But the intent is that you don't take any TWF penalties with your shield attacks. That's it. So in your example, if we assume your base attack with a longsword and heavy shield is each +8, then when TWF your attack routine could look like this: Longsword +4/-1 (1d8+x), Heavy Shield +4 (1d4+y). With Shield Master, your attack routine would then be: Longsword +4/-1 (1d8+x), Heavy Shield +8 (1d4+y).
As a side note, TWF with a Heavy Shield is probably a bad idea. If you fight with a Light Shield instead, you give up 1 point of AC and a fraction of damage, but you only take a -2/-2 TWF penalty, instead of -4/-4.