1) Changing DLM profile from 'speed' to 'standard' will not in itself trigger a DLM reset, however changing backhaul will trigger a partial reset (basically all except banding if you have a banded line). As Sky use their own backhaul you should get this partial reset. DLM however will be more sensitive to error seconds and retrains/re-syncs on the 'standard' profile.
2) LLU is essentially Sky's own equipment in the exchange with their own backhaul, without going into much detail.
3) Yes it should as DLM should be partially reset due to the backhaul change. There should only be one re-sync. Changing your login details on the DrayTek might make it ask you to restart the router to take effect, but this single additional re-sync (if this happens) is highly unlikely to cause DLM to punish you.
EDIT: Misread question 3, my "yes" is meant to say "yes it should be fine", not implied as "yes it should re-sync constantly".