At the beginning, you can not write solid structures like this
thinking more like I-Sn=I. I don't know what the right structure would be, either I-Sn-I or I=Sn-I, or something different.
because there is a connected structure within the solid piece of SnI2, that so it is solid
notice that in one form there is lonely electrons pair (SnI2) and in the second there is not (SnI4) and because of that the attraction is bigger therefore the crystal energy is huge, this is why the MP, BP... are different.