yaheyaarakil you are correct. The memory cannot be removed from the Nexus 5 it is not made to support a standard micro sd card, instead the memory is stored via the internal RAM.
However to answer your motherboard inquiry, it CAN be replaced. You can access easily by carefully prying off the back cover held by plastic clips and removing the battery and mid frame along with the interfering components.
iFixit already logged the teardown for your use : [guide|19016]
And the part can be purchased online for about 80 bucks, plus shipping of course. Id recommend searching eBay first before venturing toward other avenues.