Because this is the standard grammatical pattern for this kind of thing.
If you start with the general case without asking the "what" question it might make more sense.
こんにちはと言えばいいです。→ It's good if you say "Konnichiha." (or, You should say "Konnichiha.")
お酒を飲めばいいです。→ It's good if you drink sake. (or, You should drink sake.)
Now consider the question with "what":
何を飲めばいいですか? → What is good if I drink it? (i.e. what should I drink?)
何と言えばいいですか? → What is good if I say it? (i.e. what should I say?)
[Hello] を日本語で何と言えばいいですか → What is good if I say 'Hello' in Japanese? (i.e. How do you say 'Hello' in Japanese?)
No idea about the comparison of ~ばいい to ~ったらいい. Seems like that would be neutral.
If I wanted to be more polite, I'd change いい to よろしい. Or throw an いかが in there. Depending on who is speaking to whom.
There is another similar expression "the dictionary form といい" (e.g. この本を読むといいです), but this expression is usually not used in a question, thus, お酒を飲むといいですか sounds a bit awkward.
As for ~たら conditional, It's "past form + ら", i.e., "What should I do?" is どうしたらいいですか. しったら means 知ったら, not the -tara conditional of する. Similarly, it's 上げたら for 上げる, not 上げったら.
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