To answer Question (2):
Once you find Fx and Fy, you can use the Pythagorean Theorem to find the overall frequency F:
In looking at the source image, however, it seems like the waves are primarily traveling in the X-direction, particularly in the middle part of the image (crop the image by eliminating the top 40 percent and bottom 25 percent of the image, and then you are left with just the middle part). So that suggests that Fy should be essentially zero, in which case the overall frequency F is approximately equal to just Fx.
Once you have computed F, you can then find the wavelength:
So, to answer Question (1):
I would suggest that you break the image up into three sub-images, called top, middle, and bottom:
[M,N,P] = size(cimg);
h = floor(0.40*M);
k = floor(0.75*M);
top = cimg(1:h,:,:);
middle = cimg(h+1:k,:,:);
bottom = cimg(k+1:end,:,:);
Then, I would repeat your analysis on each of these three sub-images. I think you will find that the peaks in each image will be much more obvious. For the middle image in particular, you should see three definite peaks: one located at Fx = 0, and two others at Fx=+Fo and Fx=-Fo. There will be no major peaks in Fy except at Fy=0.