How can I determine, in what extent the fit to experimental data is good?

I have experimental spectrum in which y-axis is intensity values, and x-axis is frequency values.

Int - array of experimental intensities (y-axis).

w - array of frequencies (x-axis).

I know the view of theoretical function that must describe the obtained spectrum. I explicitly set the function in Matlab, using syntax:

fun = @(p,w)p(1).*exp(-2*((w-p(2))./p(3)).^2);

I set initial guess for parameters as:

p0 = [a, b, c];

where a, b and c - specific values which I chose as initial values for parameters p(1), p(2) and p(3), respectively.

Then I make fitting using the following code:

[p,resnorm,residual,exitflag,output,lambda,J] = lsqcurvefit(fun, p0, w, Int);

According to Matlab's help for lsqcurvefit function, residual is calculated as fun(p,w)-Int at the solution p.

After that, I find 95% confidence interval:

conf = nlparci(p,residual,'jacobian',J);

The next step is plotting experimental graph and fit function - this step is not important here, so I will miss it.

The final step is building residuals plot:

plot(w,residual,'.')

I have 5 questions:

1) Is it enough to consider 95% confidence intervals and residual plots in order to determine whether the theoretical function fits well the experimental data or not well? Or there are other quantities which should be calculated in order to say that the fit is good or bad?

2) What is criteria for that the calculated 95% confidence intervals are reasonable? For example, I obtained the mean value 1560 for parameter p(1), and 95% confidence interval calculated is 1400 and 1720, i.e. the error is +- 160. But if calculated 95% confidence interval, for example, is 1200 and 1920, i.e. the error is +- 360, will it be still good? Where is the limit? How can I be sure that the calculated 95% confidence interval is acceptable?

3) What is criteria for that the residuals plot is good? I mean, what deviation of experimental data from fit function is acceptable? Everywhere is written that residuals plot must be symmetric around zero level, but again, the deviation can be very large - is it OK?

4) I found two sites where the residual plots are treated by default for linear regression model, here are the links:

http://www.r-tutor.com/elementary-statistics/simple-linear-regression/residual-plot

http://statisticsbyjim.com/regression/check-residual-plots-regression-analysis/

So the question is why do these authors consider only linear regression model when talking about residual plots? What about non-linear regression models? For example, Gaussian function is non-linear function.

5) What type of residual plots does one need to build - residuals vs frequency, residuals vs fitted values, or residuals vs experimental intensities? Or all of them?

I will be very grateful for any help or advice.