But why 7.5v? USB supplies 5v, and IIRC the USB power source is connected to VIN.
No it is not.
Vin is connected to the on board 5V regulator, so you need 1.5 above the output voltage in order for it to work correctly.
That arrangement for two transistors is a Darlington pair and will not solve the inversion problem. Just use two common emitter circuits cascaded.
Yes that is what the LED will look like if you connect it between the RX and ground. This is because the default state of the line is high and it goes low for data. Therefore it is always on and occasionally goes off. This is almost impossible to see. Connect it the other way round, LED anode to +5V, cathode to resistor, resistor to RX.