To answer to your questions:
- the blending is just a simple pairwise alpha blending (on two images, the first one has weight 1, the second one 0.5);
- the 'expression', in the case of multi-size products, has not too much sense. Expressions were designed to work with pixel values. In the case of, let's say, B8 and B8a, since a pixel in B8a corresponds to 4 pixels in B8, it doesn't make sense to use these values in an expression (which value from B8 will it use?);
If you do the mosaic at 30m, that means you resample the bands at 30m and the resulting mosaic is no more a multi-size one, but a single size. AFAIK, the blending, in this case, doesn't work if there are NaN values (it's a JAI - the image processing library used - limitation).
For the result you've obtained for multi-size, could you please specify the products used?