can people camp there?

The Cascade-Siskiyou Monument is on public lands and managed by the Bearu of Land Management (BLM). Diapersed camping is allowed in most areas unless otherwise marked. Generally, on public lands you are allowed 14 days in one spot. The guidelines ask that whether tent, car, recreational vehicle camping to please choose a spot that is already disturbed and practice leave no trace. If you want a developed camp site most people choose to camp at sites near Hyatt Lake. https://www.blm.gov/programs/national-conservation-lands/national-monuments/oregon-washington/cascade-siskiyou

Yes

Yes only in designated area

There are places to camp such as Hyatt Lake and Howard Prairie.