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1) Did you keep loop at Rs*2 because this is the rate of the input signal to loop filter (sample rate)?
2) Updating with error is done by summing the error to the initial value or just feeding error to NCO?
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I kept loop at sample rate of Rs*2 based on my past experience. Higher rate should be better in theory, loop @ symbol rate is something I haven't tried and not sure if it works as high sideband filter(branch filter) may get in trouble.
updating means summing up fraction of error to nco word.
Remember how rtl nco works: its accumulator starts from some value (zero or so) then increments by nco tuning word. The update will add to accumulator value pushing its phase (nudging it) then you revert back to nco tuning word, this means you keep track of frequency lock after each update having now a different accum value but same inc step. eventually we hope the error becomes zero and keeps accum happy.
If you lose frequency lock then phase goes wild and out of control. I think software dsp don't have this problem as they control freq separately.
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