Wow, that's a lot of questions in a brief post!
For tight beam, especially if you want little spill, I doubt you're going to get it in a mag head with multiple emitters. And if you want light equal to or exceeding that overdriven 35W halogen (I estimate about 1000 lm), you'd want a triple XPG board or an SST-90. There are some triple XML board/optics combos available, but you might not find one with a beam you like. Packhorse has been saying he likes the 44mm aspheric from DX, and I think he's posted beamshots of it with the SST-90. A single XML is around 1000 lm, but by the time you get through your optics, you'd probably have less light than your halogen, albeit at vastly less power. XPG's are around 500 lm each, and SST-90 is probably 2000, though they claim 2200 at full power.
Basically, a tight beam means big optics. And a big LED means big optics. The physics just don't let you cheat. For a reflector, collecting lots of light into your central beam (vs letting it spill out the sides without hitting the reflector) means the reflector needs to be deep. A reflector with depth and width approximately equal lets out a good bit of spill. Your halogen is probably in this range.
For an aspheric, any light that doesn't hit the back of the lens is either absorbed by the body or reflected out as spill, so capturing lots of light means keeping it close to the LED. That means you need a short 'focal length'. I know that's not exactly the right term, but it's close enough. Short-focus aspherics are thick, bug-eye looking things. Thin aspherics have long focal length. They will give a fine beam, but lose a lot of light.
Your battery pack is probably fine, depending on the driver you end up with. Or you could reduce the weight and/or increase the runtime by switching to lithium. The 18650 is by far the most popular. I personally like the 26650. That's C cell diameter, but longer. In case you don't know, the first two digits are the diameter in millimeters, and the next two are the length, and the zero on the end means it's cylindrical. Sizes are approximate. Cells vary slightly in diameter and considerably in length. Choose a cell and get reliable numbers for that cell before counting on an exact size (cells of the same model and brand will be pretty consistent). I won't try to discuss chemistries, that's a huge subject that's been debated ad nauseum elsewhere.
You will certainly want a buck driver, since your battery voltage is higher than the LED voltage. I suppose you are used to single mode (on/off), and if you're happy with that, Der Wichtel makes a driver suitable for the SST-90. I make one that can have high/low/off modes, and drive an SST-90, but it's a linear driver, which means you have to keep the battery voltage close to the LED voltage or the efficiency sucks. It turns out that 4 NiMH cells and one LED works out great. Eight and two or nine and three also works. And of course you can parallel as many cells or LEDs as you want, so you could run a triple XML board on a 9S2P pack. Unfortunately there isn't a combination of LiIon cells and LEDs that works very well. If you want more modes, your best bet is going to be a triple board or single XML. I've not seen or heard of a multi-mode buck driver that can run 9A to the SST-90, though Taskled has some that are 6.6A IIRC. Taskled makes great drivers.
If you go LiIon, you'll want to think seriously about a 'hobby' charger. These are designed to charge a variety of multi-cell packs quickly and to treat each cell individually, as is necessary for proper care of LiIon cells. Alternatively, a good balancing protection board can be built into the pack, but it gives you less control and less information about the health of individual cells.