Although it's been too long since I listened to the drone of an EE instructor on this, I can't remember the phase angle being the important item. Tell me if I'm getting this wrong, but the two things you need to know most are the zero crossover point and direction of the voltage curve at that point. This is to make sure that you match the utility both in timing and direction of voltage. The power companies around me have a 1% tolerance on zero crossover and don't mention the voltage except for maximums. I think that's because as long as the timing and direction are ok, and you don't exceed the maximums it'll work. Your inverter should take care of that stuff for you. Unfortunately, since they are tied together and the inverter is working hard to make them the same with a tiny increase in voltage to transfer your production to the power company, it will be hard to tell if you're pushing power out or taking power in except for the metering provided by the inverter company or power company.
By no means does any of this make what you want to do impossible. I suggest that you build a basic power meter based on Trystan Lea's work with the current transformer around the power input to the grid from your supply. If it shows power coming from you to the grid, that's exactly what you want to know. Another CT around the grid input to your place will tell you net usage and a little algebra would get you all the info you want. In the US I can do the same thing, but I have to double the number of CTs.
As for the arduino being able to handle the load. I think yes. I'm sampling power many times a second and averaging the readings with CPU cycles to spare. You should be able to measure more than one thing at a time and do the necessary calcs as well. There is the caveat that you have to use floating point math and that really eats the time, but with two inputs I don't think there will be a problem, or if there is, another arduino is not too bad a thing.
Feel free to snag the code for my power monitor at http://draythomp.blogspot.com/
but remember that I have two CTs and you'll have to change the code to adapt.
As for noise, that can be a real nuisance. I recently installed a permanent magnet motor and it creates a ton of noise. My power factor dropped from almost unity to as low as 80% at times (high load) and the power line noise permeates the house and makes power line communication systems fail. Working on that problem right now, I am. However, the power savings are really going to make it worthwhile. I don't think trying to eliminate this factor in software will work due to the mess noise makes of the power waveform. However, if your measured power factor is high enough, it won't matter anyway.
However, your work on measuring the power and your systems reactance will tell you a whole lot. So, have fun with the current transformers and don't worry too much about the dangers people love to scare you with. Just turn the power off, do it, turn the power on. I'm sure you have enough common sense to handle it just fine. I'm basically an uncoordinated idiot and I managed it just fine.