Answer to "What adhesive is safe to use for keyboard rubber cap under spacebar?"It is true that you need a little bit of glue to hold it in place, otherwise it is possible (doesn’t always happen, but it can) that the rubber cap will get completely out of alignment with use and stop activating the circuit underneath. I put a tiny blob of superglue on the tip of a needle, then spread it on top of the rubber cap, so that the rubber cap would stick to the plastic key-piece covering it. This effectively keeps it in place, if you don’t mess up the alignment when you add back the key-piece. The advantage is that no superglue gets on the circuits and plastic mechanism underneath/around the rubber cap.
Answer to "replace SSD 15" in retina MBP with Samsung 840 Pro Series MZ-7PD512"Hey folks, Just wanted to say that I did exactly what @dan2h advised and I managed to replace my SSD in my mid-2012 MacBook Pro 15' Retina with a Samsung 850 EVO mSATA. It works fine. Basically you just order the Samsung SSD, then you buy this adapter and follow the instructions in this video. Don't forget to get the special screwdrivers, P5 and T5; I used this one because it was half price. The key point here is that Apple charges £490 to replace the SSD for my particular model (alternatively the OWC and Jetdrive custom-built SSDs are priced in the same range), but I fixed it myself for ~£150. Thanks to everyone whose online footprints I've followed. Hope this helps someone in the future.