Yeah or it will only be 100meg on the wan port of the draytek you can use 2 pairs for the 100 meg for the draytek and 2 pairs for your unit back to the wan facing router.
Yes id expect issues ahead. From discussions (that I understand as some are really technical)868MHz is a sideband of 900MHz. But it was believed the transmitted ie phone etc had to be fairly close to the transceiver etc