Yes, always cover them with a tarp to protect from sun until you finish the walls. Water is not a problem, but prolonged sun exposure will destroy the polypropylene material of the bags.

Once you finish the walls, you will cover them with 2-3 good levels of adobe and you will not have anymore this concern.

When you fill them, do not use dry dirt, have a moisture, like 5-10% water. Not too dry, not too wet, enough to be able to mold them in good shape. Also the moisture, once is evaporated, will leave the bag hard like a rock. Once is covered in adobe, there's no concern that will melts like sand.

You need to do your own tests, depending on the dirt you use, its composition of clay, sand etc. you have to see how much water you add or not.

When I got few days raining, I had to wait 2 days with sun until the dirt had the right level of moisture to be able to put it into the bags.

That's why this construction could take long time because not always you will have good weather. We re I live is raining enough per year so I can't build all seasons.

Moisture is very important !