Generally, Beta is the area where the propeller governor no longer governs the propeller RPM, as it has typically hit the low pitch stop.
There is a Flight Beta and a Ground Beta range present on the DH8, separated by the power lever position "Flight Idle". Flight Idle results in a prop blade position safe for flight and still noticeable forward thrust, while in Ground Idle, even some mild reverse thrust may be delivered (DISC and slightly above). In order to get to Ground Idle, one has to lift a release catch (below each power lever) before pulling the levers further backward. As going below Flight Idle is not allowed inflight on the DH8, depending on the mod status of the aircraft, the release catch will either trigger a warning whistle or be mechanically locked when no WoW is sensed.
For consequences of reducing below Flight Idle, although not on the DH8, the
accident to a Luxair F50 in 2002 may be of interest. Nothing else is to be expected from a DH8 in the same situation.
Full left rudder may be owed to a crosswind from the right. As in a jet, the DH8 shows some weathervaning tendencies under reverse thrust. This can already show up in DISC. So a bit of rudder during landing is often needed under these circumstances and would not appear out of the ordinary to me in a normal landing on all 3 legs. In this landing with the aircraft drifting to the left, I am not so sure though.