The issue here is voltage drop in the circuit supplying the saw. The saw's motor has an optimal voltage range, and will not run well if the voltage is too low. The voltage drops over the length of the extension cord; the longer the cord, the greater the drop. However, a heavier gauge cord will have less voltage drop than a lighter gauge cord.
(Voltage drop is a factor right at the receptacle, even without an extension cord - the gauge and length of the wiring from the panel to the receptacle, the load on that branch circuit, the load on your whole service, the gauge and length of your service conductors / feeders, heck even the load on neighbors service can affect your voltage at the receptacle.)
You didn't mention the manufacturer specifying a gauge for the 50' max extension cord, let's assume they figure you'll use a 14 gauge cord, and let's assume the saw draws a full 15A (which it probably does not). The voltage drop for that 14 gauge cord at 15 amps is about 3.79 volts. A 12 gauge cord 100' long at 15A will drop about 4.75 volts. So I'd feel completely confident with a 12 gauge 100' cord - one volt is negligible.
If you really want to test the voltage drop, testing at the end of the cord doesn't tell you anything because you need to see what the voltage is under load. If you really want to test this, and if you can do this safely, you could plug a receptacle splitter or power tap (rated for the load, of course) and check voltage while someone runs the saw full bore cutting wood.
This seems like a really bad idea.
First, you can't just ignore the voltage difference problem. The battery charger might have a chance of working natively or with an adapter (read its labeling) - but that's a hokey way to hook it up. The Dremel is probably beyond the ability of cheap Chinese power adapters, not least because it's a motor and I seriously doubt that overpriced $23 thing with 50 cents of electronics inside it actually does VFD.
It's generally a fool's errand to try to convert tools from one voltage/frequency to another. If you want that, then get Euro or UK tools that are 230V/50 Hz.
Further, there's the matter of plugs. USA stuff will not plug into Russian outlets, nor should it. Russia uses the Europlug (ungrounded) or Schuko (grounded). This is the same as almost all of mainland Europe. So order your father-in-law European appliances if you must.
Lastly, tools are personal. You select the tool you like, and then you adapt to it and love it. It's a thing you've got to do.
Parents don't just use stuff you give them because you gave them. You know perfectly well this is the best you can hope for. 90% of the time it goes on the shelf or in the trash. Given that you'll be giving tools of incompatible voltage, and incompatible plugs, this is 100% guaranteed in this case. If you want to do a gesture like this, either recognize it for the foolhardy, empty gesture it is and hope your FiL will feel the love... or get serious and obtain Euro appliances. Or better yet, sort out the barriers to giving a gift card at your FIL's favorite tool shop.
Best Answer
Any circular saw will cut aluminum, copper, or any other non-ferrous metal easily with a sharp carbide-tipped blade.
Keeping the cuts straight, square, millimeter accurate and not damaging thinwall parts will require some guides. My preference would be a radial arm saw, the length is easily handled with a clamp on the fence. If you don't have one, buy or rent a cutoff saw and build a jig. Time spent building it right is repaid tenfold when you cut all those pieces.
Use a new blade - the one that made your deck 5 years ago will make a mess of the aluminum. And get some earplugs too - cutting metal tubing makes noise roughly comparable to the Space Shuttle lifting off.