The jobs with later arrival times are inserted into lower-priority queues. As jobs are waiting to execute they can be bumped to higher-priority queues to make room for new arrivals. The relationship is dubious. Busy waiting can be avoided by using blocked waits.
Semaphores provide a method of blocked waiting.
The OS tracks the time with a counter, and this counter is incremented at each timer interrupt. By disabling interrupts too long it is possible to miss clock ticks, and the OS will think that less time passed than really did. This impacts programs that rely on accurate time information. Also, most OSs use the timer to trigger scheduling, so missing ticks will effect the scheduling algorithm. One way to minimize this is to ensure that interrupts are never disabled longer than a clock period.