Vicky,
The short answer is it depends. From your posts it appears that this dining issue has you very concerned, and it really should not be. Usually you can make a reservation if you must have one, but the concierge generally will stop taking confirmed reservations when they are about 1/3 full for that dining time. So one option is to figure out your schedule for the whole week, and then make reservations early on. If you want to eat when a lot of others have already reserved, you may be out of luck., but pre-planning would increase your chance of getting a table for just two. The other option is to just show up at your randomly chosen time, but be willing to wait until a table for two opens up. Or if you can't wait that long, take a table with other people. It depends on what is more important to you: eating alone or eating when you want.
If you want to KNOW that you will eat at a table for two, you might be better off taking the traditional route in the future. You CAN get a table for two in both the early and late traditional seatings, and if either of those seatings still is open for your cruise, you could have Princess switch you to trad. seating.
As for PC dining in general, I have seen it both crowded and not. It depends entirely on your fellow passengers; their myriad desires, and hard to quantify issues such as when the popular shore tour got back, what time/day the best show is, what time that particular mix of people want to get bed (or not), and even the time of the sunset. Occasionally there are reservation system problems, but IMHO that and the ship have much less to do with there being lines than the whims of the other passengers.
Hope this helps allay your worries.