Jaezell, answering your questions,
1. No, it is not necessary to have 2 stick of memory on the device. It should work fine using one memory stick only, on DIMM #1 which is the slot on top.
2. Yes, the memory supported on these devices has to be DDR3L 1600MHz and 1333 MHZ 1.35 V. The Crucial memory with part # CT102464BF160B is in the list so it supposes to be compatible with the D54250WYK.
3. I was not able to find that part # CT2CP51264BF160B on the list of tested memory. The speed of the RAM is supported and also the voltage, but I couldn’t find any other specification.
4. You already performed the recommended troubleshooting, so my suggestion will be to wait until you receive the the memory ordered and test it on the NUC. Make sure you have the latest BIOS version installed https://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=23337&lang=eng&OSVersion=%0A%09%09%09%09%09%09%09%09%0A%09%09%09%09%09%09%09&DownloadType=BIOS
If any of this make your NUC to work, please get in touch with our Warranty Department so they can help you to replace the unit: Contact Supporthttp://www.intel.com/p/en_US/support/contactsupport
Thank you, Sylvia! I went ahead and ordered Crucial's CT2CP51264BF160B kit. This was the first time I got it to boot up! So, you can scratch off Crucial CT8G3S160BM from the Compatibility list, or at least the single stick. I am still having the issue some others are with hitting F2 to get into the BIOS. This is a great box, but very picky on memory!