1. I would not loose the boot manager if I update to the 138 version, right? YES
2. Because I want to stay on 10.13 for now how do I make sure, I can boot from my NVMe SSD? YES
3. A few people said, NVMe needs to get injected under 10.13, but I am already there, so nothing to do there? For boot support, it needs to be added to Firmware
4. Do I even really "need" to update the Firmware at the moment? It's advisable. It fixes microcode and 5.0GT/s on PCIe slots
5. Is there a real advantage for me? (my tasks are Adobe CC/photo editing, a bit FCPX and video decoding with Handbreak). Probably
And how do I add it to the firmware, if I only have a NVMe drive available from which I am booting and working?
Do I need another drive, booting from there, install the firmware update, quit the installer, inject again the nvme files and boot back into the ssd?