LG G5 - strictly H850 only for /e/?

This may seem redundant and my guess is the answer is “no” based on what’s explicitly available but would like confirmation anyway.

LineageOS has builds for both US and EU versions of the LG G5 (H830/H850).

The /e/ docs show only an H850 version. Will the /e/ H850 build not work on an H830 version? What are the differences between different ROM builds for the same phone model but different country model? Will there be a higher risk of bricking if attempting to install /e/ H850 on a non-H850 LG G5?

Yes, very high risk to brick your device for ever.
Never try flashing a rom from a device for which it was designed

Yes. What reason would LineageOS have to build separate ROMs for the LG G5 H830 and LG G5 H850?

One reason will be the 4G LTE basebands:
H850: 1, 2, 3, 4, 5, 7, 8, 12, 17, 20, 28, 38, 40
H830: 1, 2, 3, 4, 5, 7, 12, 20

There is also a separate version of Lineage Recovery images and TWRP Recovery for h830 and h850.