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I prefer the first option. Buried is much more straightforward. Rest is understood but personally I think it doesn’t carry the same weight in this context. Also you can delete the first ‘and’ and just go for the comma. I would write: Only the bravest Soldiers in the United States were qualified to be buried here. Their souls transformed into little hummingbirds, guarding this land awaiting the return of their masters. I changed America to USA only because a lot of my Latin American friends will always remind me there’s more than one America 😉 but if you want something more close to the original you could write: Only the bravest soldiers in America were qualified to be buried here. Their souls turned into little hummingbirds to guard this land and wait for the return of their master(s). I think if you have master plural it implies that you are not talking about God, but someone /something else (maybe family or military superiors?) You can certainly use “rest” instead of buried, I just think buried gets the job done better, but one could certainly argue “rest” is more poetic here. Hope this helps!

I think both are perfectly correct. Every difference between them is an acceptable option, with each choice being a matter of style. If this were an exam question I would not know how to choose. 1) "To be buried" is straightforward, plain, direct English. "To rest" is a common euphemism for death and burial. It is indirect. It softens the meaning. It sounds more dignified. "The murderer buried the body." "The brave soldiers were laid to rest at Gettysburg." 2) Each passage contains two complete thoughts. They can be expressed correctly as two complete sentences, as is done in your second sentence. They are closely related thoughts. We might regard them as a single, complicated thought. They can be joined, as is done in the first sentence. Because the connecting word "and" is there to separate them, the comma is the correct punctuation mark. The choice is a matter of style. Imagine you are giving a speech. If you would naturally say it in a single breath, you might write it the first way. If you would naturally pause for a breath, you might choose the second. 3) "To guard" and "guarding" are both correct. There's no difference in meaning. Having chosen "to guard," it is best to use the parallel form "wait," and having chosen "guarding," it is best to choose "waiting," which has been done. 4) Either "masters" and "master" could be correct, depending what is in the mind of the writer. "Masters" imagines that each soldier or hummingbird has their own master. "Master" suggests there is a single master over all of them. If there is a religious implication, "master" might be better.

They are equally correct, and neither is better, just different, so which one you use should depend on what impression you want to make. The present participle and the infinitive are similar in many ways. Both can be used as adjectives, nouns, or even adverbs. Almost always, you can substitute one for the other but when you do that, the feeling changes. For example, Shakespeare wrote: "To be or not to be, that is the question." He could have said, "Being or not being, that is the question." but the feeling is different. It is almost always like that. The infinitive and present participle are grammatically similar but they are used for different purposes. Participles behave as descriptors. They paint pictures. Infinitives serve to state exact precise meanings. They convey facts and information precisely rather than provide vague descriptions. In your examples, both your infinitives and your participles behave as adjectives that modify "hummingbirds". The first sentence gives a factual description of what the hummingbirds do. The second sentence evokes an image, helping you to imagine the hummingbirds. The second sounds more poetic.

Both quite fine. The second is more succinct.