1/ Builder NPCs are a type of NPC whereas Custom NPCs are a class of NPC. Builders are considered part of the Conversation class of NPC (yeh, not the most accurate description). Conversation class NPCs either do nothing but talk or perform an in game action like building, mining, recycling, processing etc.
2/ Some buildings you do, some buildings you don't, though they're not strictly datacards (see 3/). You can find a list of which faction types can build which facilities & stations, and which facilities & stations are available to all here.
3/ Datacard makeover is currently not available, won't be until combat. Facility & Station build rights at the moment aren't considered to be datacards, what you can build is based solely on your faction type and cannot be transferred to another faction This is to prevent farming of cards. Come R&D people will be able to create new facilities, or modify existing ones, these will take the form of datacards and will thus be able to be loaned or sold.