Yes, DDR4 can function with DDR4 LDRD mode.
Yes, DDR4 can function with DDR4 LDRD mode.
Yes, it's possible to use a normal DDR4 in a desktop PC. The Kingston product you mentioned is compatible with standard systems.
NO. LR DIMMs (load reduced) are server memory sticks, regular desktop processors and motherboards don't support it. There's also ECC DDR4, these are DDR4 with extra error correction. These MAY function in a regular motherboard, it depends on the BIOS of the motherboard and the processor. For the ECC feature to work, both the motherboard and CPU need to support it. If it's not supported, such memory sticks will work like regular memory sticks. BUT, some motherboards will simply refuse to start if they detect that ECC feature. Some processors have this ECC support disabled (for example a lot of Intel processors ignore ECC if it exists). The AMD Ryzen Pro processors officially support ECC if the motherboard also support it. The regular AMD Ryzen "unofficially" support it as in AMD doesn't block it or disable it on purpose, but don't guarantee it works.