While SharePoint might not be the perfect solution for storing passwords, it is certainly possible to do this in a custom list (like managing any other info).
M: break role inheritance on list/site, remove unwanted readers/contributors/owners and give security group containing Developers read permissions
S: give selected developers (or security group containing these) contributor access
C: create a custom permission level "Add only" that gives access to add but not edit and assign this to specific users or a security group (this has been asked before)
W1: Not entirely sure what you mean, but if this means that some list items should have special read permissions, just break role
inheritance on them and assign special read/edit permissions.W2: In SPOL 2013 you have access to lists using REST. You could pass this link to your developers.