I know of certain uni research into embedded PZT material in Carbon fibre blades that optimizes flow angles at the first (big stage).
As for your questions:
1) thrust is dependent on M(dot)*(V2-V1) IE the increase in speed if the air (for every action reaction etc). If you incresed the area any more, thus decreasing the increase in velocity, you would have a corresponding decrease in thrust at higher speeds. Also, think of all that extra drag!!! Big flat round disk = lots of drag!!!
2) Gearing in some respects, multiple stage turbines (up to 3???) and yes, they have transonic blade tips on some blade sections on the outer regions. (especially in the HP compressor). Asfor variable pitch, forget it!!!! Weight penalties together with lots of complex gubbins to go wrong. Turbofans are great as they are so uber reliable. I wouldnt want to see a 3 metre front end fan lose pitch control to fine pitch and spin off the hub as it would probably rip the wing off!!! (think of all that drag and torque effects)
A lot comes down to material limitations. Work out the centripetal forces acting on the root of a CF fan blade. Its Huge. Absolutely huge. I think its similar to having a couple of tonnes hanging on the end of the blade. Thats before you even get into the forces involved as a result of work done on the Air to produce thrust. Rule of thirds from what I remember. 1/3 of the stress at the root due to squeezing air, 2/3 due to centripetal forces. Or at least thats what I last calculated at uni whilst carrying out a CFD/FEA test on an experimental blade
