I need accommodation for 1 night (no pass so can't stay in official backpacker camp).
When I was at the Backpacker Campground, I never saw a ranger or had anyone check for a permit. However, I heard from others who had been checked.
1. What are the chances of getting a spot in Camp 4 at 1pm?
Small. It's good that you're arriving Sunday, when many people leave, but it's best to be there in the morning. There's not a lot of sites:
http://www.yosemite.ca.us/maps/camp_4_w ... nd_map.png
You may be able to "hook up" with someone who already has a site, there's a limit of 6 people per site. Curry Village may have an opening from a last-minute cancellation. Check with them too at check-in time (I think it's 4pm).
2. I’ll be queuing for a JMT pass on Monday morning. Any opinions on what time I should queue? 5am?
It's open 7:30-6 in summer, so 5am is a good time.
3. Are there any alternatives to Curry and Camp 4 – in walking distance to where I queue for pass.
Not really. The drive-in campgrounds are very popular in summer.
4. If I get a pass on Monday – can I start on the Monday; it seems the pass is issued for the next day, but not clear?
If there are any permits left for the same day, yes. But they may have (probably have) all been taken the previous day (Sunday). In other words, you get in line early Monday for Tuesday morning permits. If you are flexible as to where you are going, you may be able to get a permit for the same day.
Here's a suggestion: check-in Sunday at the Yosemite Valley Wilderness Center (in Yosemite Village) and see if they have any permits left for Monday, and to just talk with them. They are less busy in the afternoon than morning when they are busy writing out permits.
Good luck!