Hockey is definitely high risk and should not be happening right now. It's sure to get rinks shut down. With the cooler weather imo many or most rinks would be able to increase ventilation by bringing in even more outdoor air, making them safer for masked, non-contact sports.
1. At the rinks I’m familiar with, masks are required when skaters (of all varieties) are off ice, but optional when skaters are on ice. From viewing protocols posted for other rinks, this appears to be common.
Hockey helmets have multiple options for face protection: none, cage, visor, and face shield. A face shield is not as effective as a mask for reducing the spread of muck, but would be superior to the other options. I think it would be reasonable to require hockey players to wear helmets with face shields.
For most figure skaters and recreational skaters, though, the options are mask or no mask. The problem with a masks optional policy on ice is that skaters can still congregate on the ice or by the boards.
2. The issues pertaining to ventilation are complex, and there are still many unknowns. I used to work in a semiconductor cleanroom. The ventilation was configured to provide vertical laminar flow of air. The floors were raised and perforated. Air was distributed from inlets on the ceiling, flowed vertically downwards toward the floor, and was exhausted from below the floor. This greatly reduced the flow of air laterally around the room, and greatly reduced the transport of contaminants laterally around the room.
In rinks I’m familiar with, the air inlets and exhausts are both mounted high up near the roof. In some systems, there are simply ports mounted on the wall. In other systems, air is distributed through vented ducts extending the length of the ice. Regardless, the air circulation is not vertical laminar flow, and the air is swirled around the rink in some complex pattern. Vertical laminar flow is not practical in an ice rink.
The nominal 6-ft social distance rule applies for people who are nominally stationary and nominally breathing normally. Skaters, of course, are flying around the ice and breathing heavily. Suppose a sick skater expels a cloud of infectious muck into the air and skates away. Along zooms an innocent skater, who looks and sees that the coast is clear ... but instead skates right into the cloud and sucks in the infectious muck. And note: The cloud persists longer at cooler temperatures, such as in an ice rink. So even if you bring in more fresh air to reduce long-term build-up of muck in the air, it might not help short-term localized spread of muck from one skater to another. To make things worse, the air flow in some rinks is not constantly refreshed, but cycled on-and-off periodically.