1. "Eat all you can" is a perfectly ordinary sentence. With hyphens it becomes an adjective: an eat-all-you-can buffet is a buffet where you pay a certain price and for that price you can eat as much as you like. However, it is normal to change the word order slightly: most restaurants who offer this kind of service would call it an all-you-can-eat buffet.
"Bottomless iced tea" sounds a bit odd -- how can iced tea be bottomless? -- but I think it is derived from the concept of the "bottomless cup". In America, when you order coffee in a restaurant or diner, you usually get as many refills as you like for no extra charge -- you can drink as much coffee as you like. Obviously, the cup isn't literally bottomless, but the idea is that it is rather like drinking from an infinitely large cup. Presumably in a country like the Philippines, a more refreshing drink like iced tea is appropriate, and so they call it "bottomless iced tea" to indicate that it's iced tea you're getting, and you can drink as much as you want. (In America, as far as I know -- an American may correct me here -- you don't need to actually write "bottomless coffee" anywhere because that is actually expected.)
2. Ask yourself this: Does it modify a noun or a verb?
3. "Go ahead" is what you say to somebody when they are not sure if they should or may do something. For example:
"May I smoke?"
"Yes -- go ahead!"
That means: Yes, you may; please do so.
When you want to leave the office, you can say, "I'm going home now." If you are not sure whether you ought to go home now (maybe someone needs your help), you formulate a question: "Is it all right if I go home now?" (and your colleagues may answer, "Of course, go ahead"), or "Do you still need me here or can I leave now?". And when you do leave, you say something like, "Bye!", "Goodbye!", "Good night!", "Have a nice weekend!", "See you tomorrow/on Monday!" or whatever is appropriate.
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