Error plot between known and estimated data
Your formula/method for computing MSE between estimated and known inputs looks good to me. For symbol error rate you could use something like a Hamming distance which simply counts the number of times the estimated symbol is different from the actual symbol. In your 10 symbol example the error rate is 4/10 i.e 40%.

In Matlab you can do something like:

symbol_error_rate_percent = sum(u_hat ~= u)/length(u) * 100


You can compute this for $T$ trials and average them, like you did for MSE. Or you could just make a really long sequence of length $10 \times T$ and do it once.