1] VOLTAGE: 3.6V or 3.7V - 18650 Li Ion Batteries
All single cell lithium ion batteries are going to be 3.6-3.7v. There are applications where multiple cells will be tied together in series. This will result in voltages that are multiples of 3.6-3.7v. So as long as you match the number of cells and approximate mAH you should be fine.
2] Possible Voltage Shortage?
The voltages and battery life responses for all batteries are going to have slight difference. For the most part this won't matter. Most projects that use batteries are not terribly voltage dependent. They will either boost or regulate their voltage to get the voltage they want out, or they will be able to run at a wide range.
As a note, "Shortage" in this context usually means you are creating a short across your battery. Might want to be careful with that terminology.
3] Fundamental Reason for this Voltage Range
I am not an expert on this, but I know it deals with the chemistry of the battery itself.
4] Parallel Cell Charging - One BIG Li-Ion Battery Pack
This can be done. There are some issues that can come up when doing it. This might be worthy of a question by itself. If you do ask, might want to ask if the same can be done for packs in series.
5] Charging... How?
Same as previous answer.
Are you aware that a cellphone charger, although it is called a charger, is actually NOT a charger ?? It is just a power supply. In all phones there is a chip that sits between this adapter and the battery and this chip takes care of the actual charging. Did you know that Lithium based cells can EXPLODE / CATCH FIRE when not charged properly ?
Please get a proper charger circuit like this TP4056 based module it will fit all your requirements, has indicators, you can set the maximum charging current with a resistor.
Be safe, just get such a module it costs almost nothing.
Another item to consider if you only charge 18650 cells is this one.
Best Answer
The cheapest way it to get a CC/CV Power Module from ebay.
For less than $2 you could get a good one which looks like this:
For less than $6 you could get a fancier one which looks like this and which allows to monitor the charge better:
If I needed to charge many cells I'd get 10-20 of the cheaper modules and create a rig with those. They have decent LED indicators for how charge is going.