1. You can register for any subject within your course, provided you meet or exceed the requirements for a given subject (E.g. some year 2 subs have year 1 prerequisites, some do not).

2. Depends on the course. I'm in the BPsych course; one year runs thus: long-short-long semester. First semester (long) starts 2nd week of January (usually), 2nd sem (short) end of May, 3rd sem (long) end August/beginning September.

3. Long semester : 15 weeks; 14 weeks lectures/tutorials, 1 week final examinations (Mid-term examinations week not counted, nor is the week-long BPsych study break)

Short semester : 8 weeks, 7 weeks lectures/tutorials, 1 week final examinations (Again, mids and study break weeks not counted; some classes go on during mids week, some do not, depends on your subject/lecturer)

Ask your seniors; without information about your course I can't tell you anything else that's useful.

Ease of entry, depends on the nature of your preferred postgrad course. A BPsych alone isn't a sufficient indicator of whether you'll make it through the screening processes. There are terms and conditions, of course, CGPA requirements being one that comes to mind.

Sorry to not be of more assistance, but a general question can only get a general answer at best.