Yes, the voltage listed on the bulb is the nominal voltage and it will work perfectly on a 120 volt circuit.
No, it is not safe. The lamp is rated for a certain voltage, 125 volts, and supplying 240 volts to it would exceed design safety margins. It is not just a case of changing the bulb. It would be a different story if the lamp itself were rated for 240 volts, but that's not what the question said.
You tell yourself the 125 volt receptacle is a 120 volt receptacle. They're the same thing.
yes
NO! There are 50 bulbs in a 2.5 volt string or 125 volts. There are 35 bulbs in a 3.5 volt string or 122.5 volts. If you put 50 3.5 volt bulbs in you will have 175 volts and most american house have 110 volt service so you will have dim bulbs.
If the plug fits you are good to go. The typical range of residential voltage is from 110 to 125 VAC. You are fine within this range.
125% of 120= 125% * 120= 1.25 * 120= 150
Yes. The 125V is the rating of the bulb, which indicates the voltage it was manufactured to withstand. The voltage it is actually operated at does not have to be precisely 125V; As a matter of fact the voltage coming from your wall socket can vary from as low as 100VAC all the way to above 125 VAC. So, screw that bulb in and light up your world!
-120
125 x 120 = 15,000
I saw a used one on eBay recently for $19.95.
Sure
Yes.