1. Unprotected 14500s have the same size, protected 14500s are a bit longer because of the protection circuit.
2. No.
3. Most flashlights that support 14500s will also support AAs using a boost driver (probably won’t sell well to the general public otherwise), but not all of them. Not all flashlight that support AAs will support 14500.
An considerable issue with flashlights that support both 14500s and AAs is that many of them don’t have LVP (low voltage protection, i.e. the flashlight should refuse to work at all between 1.9V and 2.5V to protect the cell yet being compatible with AAs ). If the flashlight doesn’t have LVP, it will easily over-discharge an unprotected 14500 cell to about 0.8V if kept turned on unattended. It’s dangerous to recharge a over-discharged cell so you should discard it. If the flashlight doesn’t have LVP it’s better to buy a protected 14500 to prevent over-discharge if the light supports the extra length and the current isn’t too much.
4. I don’t know. The voltage of a fully charged 145000 is a bit higher than 2xlithium AA, it isn’t assured that all drivers will support it. If it goes into direct drive it may be too much current for the led or the driver. Since even the highest capacity 14500 (the 1000mAh Vapcell 14500 ) contains less energy than 2 eneloops you will notice for sure a reduction in runtime even at the same brightness.
5. There are multi-chemistry chargers. Even a $6 Zanflare/Littokala Lii-100 can charge 14500/18650/NiMh AA. If you want to charge several cells at once the price increases a bit. If the charger allows to select the charging current, for 14500s is better to keep it low, at 500mA or less to reduce the stress on the cells.