@Boycie83
Basically
@bagman gave you all the answers, i'll try to be more detailed.
1. Charger-wise, don't go above 1 to 1.5A charging currents, i explained that in my post above yours. It would be ideal if you had a separate dedicated charger, because there's no guarantee that the built-in charger will equally charge both cells. Nitecore, XTAR, Golisi, Liitokala are charger brands favoured by vapers.
2. Generally mAh's are about the runtime, but there are cases where the circuit board regulates the power, so in your case, if you don't know the specs of the torch, i can't really tell. I assume the cells are connected in series, so if LED's are working at 7.2V it goes down to more light output and the runtime equal to mAh's of one battery cell, and if they have a step-down converter PCB which ramps the tension(voltage) down to 3.7V, then it's less light output (assumed with same number and type of LED's) and runtime equal to the sum of the mAh's of the two cells. I stand to be correected at this, since "assumption is the mother of all effups"

3. Basically yes. you'll get noticeably longer runtime with 26650 cells vs any smaller dimension cells. The first two numbers are width in mm, the second two are length in mm, the zero on the end is decoration (looks cool)

4. If the batteries are of an unknown brand, unbranded or have "xyzFire" in the name, avoid using them in anything.
As for battery recommendations, avoid everything with "fire" in the brand name or unbranded. Cells with built-in protection circuit boards can be 2 to 3mm longer than unprotected cells, so be aware of that, it may happen for you to not be able to close the compartment.
Good brands are Samsung, Sony Murata Konion, LG, Sanyo, Panasonic, and most other brands like Eneloop, Golisi, Xtar, Soshine, Keppower, Vapcell are rebranded ones from the first mentioned manufacturers. If you decide to go for 26650 cells, keep in mind that they are getting harder to find because 20700 and 21700 cells are slowly pushing them out.
A very good place to learn about Li-ion batteries are vaper forums, and here are some links to a well regarded expert on batteries, who has a LOT of reviews of various cells and brands:
http://moochreviews.com/home/
https://www.e-cigarette-forum.com/blogs/mooch.256958/