NullReferenceException when deploying Sharepoint feature from Visual Studio...

... but not when you deploy it from the Sharepoint web interface. Its likely that you use SPContext somewhere. Instead you should use the SPFeatureReceiverProperties exclusively to obtain the current site or web.

Items not showing in Sharepoint despite read permissions.

I was desperatly looking why some one else who had only Read permissions couldnt see certain items in the Style Library. As it turns out when the content approval is turned on then the items have to be published even after checking them in to make them available for everyone.  Its a smart feature so that you can review minor versions without everyone seeing drafts, but on this occassion I turned it off.

ShowInPicker in BCS Model is ignored

Rather than trying to hide TypeDescriptors you need to only apply  ShowInPicker="true" to the ones you want to show. If none of the TypeDescriptors has ShowInPicker=true then all TypeDescriptor are displayed, even if some have ShowInPicker=false