I'm glad you were able to more-or-less resolve it. I'm sad that I wasn't more help.

If you figure out what went wrong, please post it here. Someone in the future might (figuratively) kiss you on all four cheeks for solving this problem for them.

No problem Richard, you helped me to learn more about LVM and the tools used to administrate it.
I have used Linux and Slackware since the late 90s and also with custom kernels nearly since the beginning but never have used LVM/LUKS before. The problem here was that "deprecated sysfs features" was enabled in my kernel which changed what is in /sys/class/block to be in /sys/block - and obviously that breaks LVM.

With enabling this: LVM can not be found as described before
Disabled deprecated sysfs feature: everything works

What I still don't get:
With this feature enabled in the kernel it doesn't boot with / on the LVM within LUKS. However, moving the whole / filesystem including this failing kernel and initrd to a physical partion on USB and boot from there, then it allows to open from within the running system with the 3.16.0 kernel with the deprecated sysfs feature to open the LUKS and LVM devices. So seems somehow later in the boot process the needed device structure for LVM is created even with this feature enabled.

Anyway, without this deprecated feature everything works and since I forgot why I put this in in the first place a long time ago (it was also in other kernels I compiled before...), I just leave it out now and for the next kernels.

Thanks everyone for trying to help!