The soda bottles the way you want to use them are mini greenhouses and would be too hot to leave on all summer. Once the leaves touch the sides they will burn. They are great for warming the soil, protecting plants from frost in the spring and for starting frost hardy plants earlier. I don't know what made you think you could plant closer if you used them. A plants roots need the same amount of space whether or not they are covered with anything. If you want to have more room in the garden then think of having some vining plants and have them grow upwards on trellises or chicken wire secured on sturdy posts. You can make a teepee with bamboo poles stuck in the ground and tied at the top. On these you can grow cucumbers, peas, pole beans, and vining tomatoes--make sure they are vining not bush varieties. I'm sure there's more but I can't think of them right now. Maybe someone else can add to my list

Went up to grocery store to buy a few items and to get steamed spiced shrimp for tonight's dinner. Next to the seafood counter was a lady giving samples of some fried shrimp product..she asked if I wanted to taste them..I took one and my eyes went to the trash bag beside her card table set and there was an empty 2 litter soda bottle in it. I asked her if that was trash and if I could have the empty bottle..she looked at me and of course said Yes so I felt I had to explain to her why. She said she'd never heard of such but she'd have to tell her Mom. So folks, keep your eyes open; you never know when you'll come across a free WS container.

As long as they aren't kept in direct sunlight, I don't think it should be a problem in your zone. I've used the baggie method more often for tenting and the only times I've roasted ones due to heat was when I forgot about them somewhere where direct sunlight got on them for too long.

I've planted my cucumbers over a week ago. They were wintersowed and growing already outside. I covered them with bottles, same as you describe. I've mulched around with shredded leaves pretty high. So far, so good and they survived a few quite cold nights. I know, it's crazy early but everything sprouted so early. I want to do this with a couple of tomatoes and sunflowers. I have to wait till weekend, no time right now.