I've never thought of using low-temp solder as a fuse before ...
You can always use a variable pot, not as elegant as pwm control, but the amount of wasted power (say, maximum of 5V@0.2A = 1W) is a negligible amount of heat for such a physically large component to dissipate. This approach creates a tiny amount of electrical noise (in the form of impedance oscillating at half the fan rpm frequency), but any not-cheap-crap mobo and PSU can easily clean and filter it out. I wouldn't personally worry about any of these things unless the resistor/pot is part of a fanbus that drives more fans or fan power.
My Noctua "Low Noise" adapter has a 70Ohm 1/4W (probably 68Ohm with loose tolerance, I haven't removed the heatshrink), and the "Ultra Low Noise" adapter has a 120Ohm 1/4W.
My mind says Technic, but my body says Duplo.
I ended up using a 1/4W one and it worked just fine thanks though!
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