1) Given that you're using regulators to power your downstream system, you could get away with simply connecting a Li-Po-compatible battery charger to your +ve & -ve buses, because the charger isn't going to apply any more than 4.2V per cell-in-series (i.e. 8.4 Volts). But bear in mind that when recharging, the Li-Po charger will need to be rated for the power demand of both charging (at the rate you choose) AND the load (assuming you want the device to work while charging).
2) Certainly. Many options. Do the math on the max recharge current needed to recharge this behemoth of yours within the time period you need. Google. Just know that putting 9 cells in parallel is non-trivial, make sure you initially connect them together in a fully charged state, and to use BPMs on each cell. Having 2 such batteries in series to get you 8.4V (full, down to 6.0V flat) suggests 'cell balancing' would be ideal, but whether you can find a charger within budget that will do balancing at whatever your system current draw is, can't be gleaned from your question.
3) *Always (*almost). Li-Po are fickle and deadly mistresses. Anything above 4.25V (check datasheets for your particular 18650s), anything below ~3.0V (check datasheets), current's in excess of their stated Maximum-xC-discharge-rate, will shorten the life of the cells well beyond reasonable expectations. That's why cell/battery protection modules/PCBs are typically added to 'loose' Li-Pos, to protect hobbyists from what most of them don't understand.
4) It's nice, but its 'power path' feature probably isn't needed in your application, and if it's too expensive, then look for simpler recharge-only alternatives, which comes back to Question-2.
5) as someone commented above, you also want 'under-voltage lock out' (UVLO) between battery & voltage-regulators (or Vregs with settable UVLO threshold voltage), to stop your system bleeding the battery dry below the safe level of ~3.0V/cell.
It's hard to be specific or more helpful without (a) knowing a lot more about what's being powered & for how long, & (b) finding someone else willing to engineer this for you properly.