My two cents. And that's all it's probably worth, at best.
1) More sweep creates boom strike and vibration issues I would imagine. If the blade is too stiff, or head speed to low, it may strike on positive pitch maneuvers (trailing edge pushed lower in the disc due to sweep). The more pitch applied also the greater flutter induced by the sweep moving through the air at high angle of attack, meaning vibration. Not stiff enough (allowing flex toward the tips to avoid strikes) and you lose efficiency improvement, get more flutter and vibration, etc. and you increase potential for strikes in negative pitch. With the low profile DFC heads so popular now I'm kind of surprised they're trying this at all. Maybe they recommend smaller blade lengths than normal with the swept tip design since you will be getting more effective lift with the swept design. That could help reduce some of the strike potential.
2) If I'm understanding what you're describing, they do this on fixed wings to reduce tip drag and provide tip stability. They also do it on those giant ceiling fan blades for the same reason. However, they cannot do it for a helicopter blade. These surfaces that help provide stability to fixed wings and fans that don't move anywhere would create turbulence and instability in the forward and aft positions of the rotor disc in forward flight. Also, on a variable pitch blade the efficiency gains in positive pitch would be harms in negative pitch, again resulting in uneven performance between positive and negative pitch uses. If you try to achieve the tip stabilizing upward curve (in positive pitch) with flexible blade designs (which would flex to create it in positive or negative pitch) you end up sacrificing overall blade lift and in negative pitch you again run into boom strike issues. On slow moving, always upright (or at least always positive collective) helis this might be made to work, however forward speed is your tradeoff. So can you get increased efficiency at an airspeed high enough to make it worth while? My guess is not, or else someone would be doing it already.
3) Twist? Like in a propeller? You could do this to give you greater lift efficiency in one pitch direction, but it would hurt the opposing pitch direction and it would hinder forward flight dynamics. Setup would become more complex. Requiring negative pitch to achieve neutral lift and possibly varying negative pitch depending on head speed. The further out the blade the twist angle extended it would hamper efficiency in forward flight and screw more and more with the lift efficiency of the aft part of the rotor disc in forward flight, much like high lift efficiency fixed pitch blades do. The less it extends out the more flat the profile is and the less lift enhancement you get.
There may be a balance that can be reached for specific applications or even for general purpose hovering and forward flight only helis, but that's not where the CP heli market is at right now. Smack is where its at and I don't think there's a twist profile that makes sense in that use case. And even in lower performance flight models, the twist is going to create vortexes more so than in a flat blade design, which the aft of the rotor disc will have to cut through causing lower lift efficiency over the bulk of the blade surface, not enough to compensate for the increase toward the center. Maybe for a hovering specific application, such as on the osprey where they use props for vertical takeoff and auto-rotors with low airspeed potential, or heavy lift helicopters with slow airspeed requirements, but for general purpose or performance oriented flight, I don't think it makes sense.
