Softfun

In the Bleed tab under File - Document Setup... you can unlink the bleed dimensions by clicking the chain, and then set the Inner bleed to 5mm and the Outer, Top and Bottom values to 3mm:

This won't change the size of your document - it will only take effect when you export the pdfs as pages.
This will depend on the number of pages in the product and the weight of the paper. I think you have to take the printer's word that 5mm will be correct and will give them the tolerances they need to avoid this. Why not send them a test?

I used to duplicate full spread images, crop left and right parts as separate and nudge them a bit to left and right to compensate the part eaten by perfect bind. How much nudge (or doubled image area) is needed depends and only experience (or maybe your printer) can tell. 
Most people don't care and let a slice of photo to vanish. Most people also check that there is nothing important details in the middle.