Well, you can't boost with a linear regulator. To boost up a voltage, you'll need a boost converter which is a switching supply.
No. you cannot put regulators (switching or linear) in parallel with each other for higher current output. They will attempt to regulate each other which will result in a problem. There is, however, no reason each regulator can't power its own set have already. All power distribution networks are based on that principle.
When dealing with LiPo batteries, my preference is to use off-the-shelf charge controllers. My go-to board is the Adafruit PowerBoost 500 and 1000. The boards have a LiPo charge controller and a boost converter that outputs 5 volts.
For large LiPo batteries like these, a fuse would be a good idea.
The 5 volt input on a Pi is a buck (switching) converter that generates the 3.3 volts used on-board. A common trick used in audio is to boost high and then use linear regulators from the boost voltage. So in this case, a boost to 8 or 9 volts then use a linear 5 volt regulator. I would not, however, use a 7805. I would look at something more modern like the NCP1117.
FYI. Switching regulators operating in the 100 kHz to 2 MHz range.
10 Amps or 50 Watts? Such a supply is non-trivial to design. I would recommend buying an off-the-shelf supply. Or at least until you've designed some lower power switching designs to get some experience with them.