[*]1. It seems like everyone plays a Brute. Why play a Scrapper, or a Tanker for that matter? I can see Stalker brings something a little different to the table, but Brute seems to do what those other two melee ATs do, without any of the weaknesses.
In general, Scrappers will out damage Brutes, but Brutes having a higher damage cap means that isn't always the case. So with the right buffs or ideal circumstances, the Brute can pull ahead. Tanks have more HP and hold aggro better than Brutes. Brutes are very good at what Tanks and Scrappers do, but they generally don't do those things quite as well.
[*]2. Why would I play Rad/Elec Defender when I can play Elec/Rad Corrupter and always be scourging?
Defenders buffs and debuffs are stronger than Corruptors since they do less damage.
[*]3. Sentinels. Are they really just intended to be solo friendly Blasters? Is that the whole appeal? Why would you want to be less potent and less vulnerable Blaster and a less tanky and less melee Scrapper?
I have no experience with that AT.
[*]4. Is it futile to try and compete DPS wise with anything other than Bio Armor? Offensive adaptation seems like a huge balance oversight. Or is that just the result of the end-life power creep of a desperate game?
Bio Armor definitely gives the most damage for a secondary
[*]a) Or Titan Weapons for that matter? Or TW/BA?
TW absolutely does the most damage(at least for Scrappers, I assume it's the same or similar for the other ATs). That being said, it's an extremely slow set without momentum and it takes a significant IO investment to reach its potential. But when it gets there, TW/Bio does stupid insane damage.
[*]5. How is Psionic Melee? I always thought Psi damage was super underrated since it is rarely resisted, but that seems like it never came to fruition as a mechanic? And of course the Mass Levitate does smashing...
I don't know, but I just made a Psi/Fire Scrapper, so I'll know soon.
6. How do my old builds hold up at a glance?
[*]a) Seems like Broadsword Scrapper has been outshined as the hard hitting primary. Or should I just play a BS/DA Brute :/
Broadsword is still perfectly useable, but you are right, it is outshined for damage. Most sets do more. If you want to use a sword, Katana is a good option since it's better than Broadsword in every way.
[*]b) Similar to my question above, would my Rad/Elec Defender just end up doing less damage than an Elec/Rad Corrupter without any trade off after considering all the late gamer Enhancement and Incarnate potential (most of which I don't fully understand). And does Sonic Attack just outclass everything since it has -res?
Sonic Attack isn't as good as the others, unfortunately. Your Rad/Elec Defender is still good, especially Rad.
[*]c) Is AR just outclassed by Beam Rifle?
Not positive, but it's my understanding, yes Beam Rifle is better.