Do they block all access to the upper deck, or can fans run up to retrieve a foul ball or have a quick "intimate" moment under the tarp?

Looking at their seating chart, they do sell three sections in home plate--those directly behind home plate. (The "3D" seating chart shows this much better.) Whether or not you can wander around up there or if they have security preventing that, I couldn't say.

EDIT: To ask, are the seats literally tarped? I know they untarp them for the Raiders' games, of course, do they put the tarps back every time?

Do they block all access to the upper deck, or can fans run up to retrieve a foul ball or have a quick "intimate" moment under the tarp?

It's easy to get to them. Fans are dissuaded from entering those areas only by plastic chains.

Whether or not you can wander around up there or if they have security preventing that, I couldn't say.

In order to get to those seats behind home, you actually are forced to walk by the entrances a number of tarped sections. There's no purpose to wandering the area behind those seats, which is just a lot of cement and closed off concession stands. You could get to the tarped seats in an instant but I imagine you'd be spotted pretty quickly. Having never done it, I don't know how tight the tarp is or how much fun you could have under it.

Those upper deck seats behind home are actually the best tickets to get - during the regular season they are $14 and it includes a $6 credit for food/beer.

I'd agree with 20, 39 and 42. I was able to get 4 seats for Tuesday's home game during the ninth inning of the last Texas game around first base on the field level. For $55 each. Just clicking around there were tons of tickets at that time.

I agree with 20, 39, 42, and 43. Not only was it authentic frontier gibberish, it expressed a courage little seen in this day and age.