My kids aren't into Lego or play mobile but my DD1 and DD2 play with Sylvanian Families and the parts are tiny! We store them I'm big plastic boxes that you can pick up at B&Q etc. Maybe Argos? Really useful and they usually come with lids.
We have all the lego friends in one trofast box. Other sets are in clear plastic lidded boxes. I second the samla idea, though ours are just the biggest tupperware I had. The boxes are stacked, end on, on a bookshelf. I cut out a label from the original packaging showing a pic of the assembled piece or general label like 'chima', and stick it inside the box facing out. But just having the clear boxes is enough really.
I think semi-organised isthe way to go if you want to keep the sets to make. Keep all the princess stuff together, and the car kits together etc. I prefer the idea of chucking it all in together for more creative making, but DC would insist on doing the kits anyway and they'd just get more frustrated looking for the bits.
That's an excellent idea about the instructions! We have a lot of Lego, and we insisted it was all in together (pieces sorted by type and size, using sanka and Ziploc bags) however my youngest insists on building particular sets over and over again, so I'm thinking I should now store certain sets whole, as sets. Would probably make us all happier.