I don't know the American national anthem, but based on the context, I would say that "heaven rescued land' means land that heaven (or God) has rescued - presumably from the perfidious English. So as it has already been rescued, you need to use the past tense - rescued. "Thus be it ever' is an old fashioned way of saying - may it always be so. Don't forget a lot of national anthems date back a century or two and some use language that has fallen out of use in modern day. Also, some were based on poems of the time, which give them an even greater tendency to use language that might not appear in daily use.

1) The larger phrase says this: "may the Heav'n rescued land praise the power that hath made and preserved us a nation!" If you understand the phrase: "may the land praise the power that hath made and preserved us a nation!" then you can add the phrase "heaven rescued" as an adjective to describe and modify the subject "land". It was a land that was rescued by heaven, a heaven-rescued land. 2)The word "thus" means "this way" or like this". The phrase "thus be it ever" means "it should be this way forever" or "this is the way it should be from now on". It is an unusual sentence order for English, but sometimes used for effect. Another example would be to say "black is the night".