What would be the membrane's form if phospholipid tail lengths vary?
Some the scenarios depicted are not energetically favorable and will lead to very unstable membranes if at all. 

In the presence of many short-chain lipids, where the difference in chain lengths is very drastic, it will become more favorable for the membrane to reorientate some of the short-chain lipids such that the hydrophobic gaps/pores in your cartoons convert into hydrophilic ones with the head groups forming the pore walls (see fig. below).



If the difference in chain length between the two lipids is not much, the bilayer will adjust its thickness to minimize the hydrophobic mismatch. In such cases, the bilayer will be thinner in some regions. The possibility of interdigitated bilayers is also very much there and will depend on the choice of lipids in question. Interdigitated lipid membranes have been reported before and you can look it up.