Should I run 1/2“ pex from a manifold to each fixture or 3/4”

You are definitely running a risk of freezing these lines in an unheated space. It is never wise to run water lines in an outside wall or above an insulated ceiling in an unheated space.

Perhaps you can do one of the following:

1. Run the lines under the floor in the basement (not unheated crawl space) where freezing will be much less likely.

2. Run the lines under the insulation in your ceiling next to the heated drywall. If you have to drill holes in ceiling joists, be sure they are at least 2 inches from ceiling to prevent accidental puncturing from drywall screws. Apply the insulation over the lines.

3. Thermostatically controlled heat tape can work, but be absolutely sure it is installed properly and if possible use a heat tape that you can use pipe insulation over. Not all heat tapes allow use of pipe insulation due to overheating of the PEX.

4. If you do have to run in this area, be sure there are no drafts that can come to bear on the tubing. Freezing is always faster if a cold draft blows on a water line.

I wish I had a foolproof method for you to use. Just be cautious, monitor the conditions and have a water shut off handy. Although PEX will handle a lot of freezing without bursting, a leak is going to be a very expensive fix and a mess to clean up. Good Luck

The circular loop would only be for the hot water side. It is called a recirculating line and it is in place so when you turn the hot water on the line will not have to purge the cold water first, the hot water will be there at the ready in seconds. It is more expensive in energy costs since the hot water is always moving through the pipes and loosing a little heat which would need to be maintained.