I wouldn't worry too much about safety if you aren't going to measure anything other than batteries but I believe that you should buy an instrument rather than a disposable item. My suggestions are either the UNI-T 61E or the Fluke 15b. Both are $60-80 and quite decent multimeters. I have a 15b...
Yes, AA's for me. They have plenty of runtime and are more than bright enough. Also love how Eneloops have low self-discharge so don't need to worry about that either.
Just to say what others have said in a different way ... The general principle is that if your device needs 12V at some current rating, say X amps, and your battery can supply it, you will be fine. You just need to make sure your adapter supports more than X amps.
Unfortunately, I don't know the answers to either of your questions but selfbuilt's review says flat-tops are ok (http://www.candlepowerforums.com/vb/showthread.php?381869-Thrunite-TN12-2014-(XM-L2-1x18650-2xCR123A-RCR)-Review-BEAMSHOTS-RUNTIME-VIDEO). The only 18650's I have are Nitecore...
Looking at ebay, you can get the TN12 for $50 with free shipping. You can also get 2x 3000mah batteries and a Nitecore i2 charger (sorry I thought $50 would be plenty for the d2 + batteries but that doesn't seem the case) for < $50 with shipping which makes the package just within your budget...
Thanks for an excellent review. I measured the D2 charger and found no trickle after it had completed charging. Details have been posted in this thread http://www.candlepowerforums.com/vb/showthread.php?389523-NITECORE-Power-%96-The-D4-and-D2-Digi-chargers-and-NITECORE-s-li-ion-cells.
I have a D2 and was interested in the trickle issue raised in HKJ's excellent review of the i4 charger at http://www.candlepowerforums.com/vb/showthread.php?387785-Test-Review-of-Charger-NiteCore-Intellicharger-i4-2014. I charged up a partially discharged 1600 mAh Panasonic NiMH battery and was...
How about the Thrunite TN12 which uses 1x18650 and outputs 1050 lumens and a Nitecore D2 battery charger? That with a couple of batteries can be had from ebay for well less than $100 Aussie. I have the TN12 and D2 charger and both are very nice. Not sure about the scoped .222 mount though.