Does this sound right? "flowing outward all around it for miles." Or should I take out "it"?

Also, is it still bad grammar to end with a preposition? Should this: "there would be no doubt about what they were sent for" be "there would be no doubt about for what they were sent". Is that right? Is that what I should say?

To answer your first question, we need to know the context. To what does "it" refer?

My opinion is that you should avoid ending with a preposition when possible, but not when it makes the sentence awkward. Clarity should be uppermost in writing. Sometimes, re-writing the sentence is best. In this case, again we need to know more. Were they sent to some place, or were they at another location and their presence needed at the speaker's location? If they were sent away to do something, the sentence could be, "There is no doubt why they were sent". If they were needed at the speaker's location (sent for as in He sent for needed supplies to continue his journey.), then you could say, "There is no doubt as to why they were needed" or, "Why they were sent for is obvious" or "The reason they were sent for is obvious."

The idea that you can't end a sentence with a preposition is one of those artificial "rules" of English dreamed up by snobs who wanted English to be like Latin, and kept alive by half-baked know-alls.

'It was John Dryden, the 17th-century poet and dramatist, who first promulgated the doctrine that a preposition may not be used at the end a sentence. Grammarians in the 18th century refined the doctrine, and the rule has since become one of the most venerated maxims of schoolroom grammar. But sentences ending with prepositions can be found in the works of most of the great writers since the Renaissance. In fact, English syntax not only allows but sometimes even requires final placement of the preposition, as in We have much to be thankful for or That depends on what you believe in. Efforts to rewrite such sentences to place the preposition elsewhere can have comical results, as Winston Churchill demonstrated when he objected to the doctrine by saying “This is the sort of English up with which I cannot put.”' www.bartleby.com

quote:

Does this sound right? "flowing outward all around it for miles." Or should I take out "it"?

Well, only you can answer. Does "flowing outward all around for miles" convey what it is that you want the reader to imagine? Visualize the scene as you imagined it when you thought of it. Does the absence of a precedent for 'it' matter? If not, you don't need it.

quote:

...is it still bad grammar to end with a preposition?

The grammarians who established that rule were trying to model English on Latin. It can be done, but it sounds labored. (cf. "Up with which I will not put," as previously cited. )
Each language family has its conventions. If French had been the idealized 'perfect language' upon which the grammarians had decided English ought to be modelled, we would be wondering about the following pronoun:

"Do you like this table?"
"Yes, I think she is very pretty."

I think I've determined I should take out 'it'. I refer to what it is in a clause before this one, and I don't think I need to say what it is that it's going all around again. Doesn't seem necessary either.

For the second, these two people were sent to a location without knowing what they were looking for. They were told to dig at a certain spot and that they'd know that what it is they find is what they were sent to pick up. Maybe that is too unclear. I could just say "there would be no doubt about what they were sent to uncover." Does that sound good? Does 'about' not need to be there? Or maybe replace it with 'as to what'

If the people doing the digging are the ones that won't have a doubt, then, "They'll know when they find what they were sent to get" seems to work. Even "They'll understand when they find it."