I strongly suggest, as in the link Sarah provided - practice first, a lot. I’ve never got on with de-soldering braid, but others seem to find it works for them.
NO NO NO!
You will more than likely lift the copper track off the PCB and that won’t be repairable.
Yes, it is fairly tricky. First, your soldering iron. Do you have one, or do you need to buy one?
If you’re buying, and if you can afford it, get one that is temperature controlled, and get the smallest bit available (the ‘bit’ is the tip that actually does the soldering), get a small coil of solder and a solder sucker - I use the ‘piston’ type where, when you press a button that releases a spring that pulls the piston up the cylinder and sucks air - and the molten solder - into the cylinder. I have seen the “rubber bulb” type - never used one but they don’t look good.
Learn how to ‘tin’ the bit the first time you switch the iron on, and how to clean it (you can use a damp rag or a damp natural sponge to wipe the bit while you’re using it).
Now get a small piece of stripboard or a discarded piece of electronics with a p.c.b. with components you can get at, and practice getting solder off connections with the solder sucker. You’ll need enough molten solder on the bit to spread over the area of contact to transfer the heat and melt the joint, before you suck the whole lot (hopefully) off. When you are confident, have a go at the emonTx4. You’ll probably find most but not all the solder comes off easily. If there’s still a bridge but not a raised lump of solder, the technique is to wipe the tip of the iron along the gap between the pads reasonably quickly, hopefully dragging the solder bridge with it. If it doesn’t come good the first time, give it time to cool and the glue holding the copper track to the board to harden before you try again.
Speed is of the essence. The iron must be hot enough to melt the solder instantly, in a small fraction of a second, high temperature for a short time is a lot less bad than low temperature for a long time.
Not that I know of. Nor can you (as far as I know) read pulses on the USB input - they need to be converted to data, a number, first. At the emonBase end, you will be using the GPIO pins directly. This is a standard RPi connector. All I can suggest - if you don’t want to cut the RJ45 plug off the optical sensor - is to get a RJ45 breakout socket and make up a short lead yourself (now that you know how to unsolder, soldering 3 wires should be easy).
I think what you need to know is either here: Multiple Optical Utility Meter LED Pulse Sensor connection to EmonPi - #2 by Robert.Wall or in a link from there.