In American English, what part(s) of speech are "or else" and "otherwise" and why is "otherwise" more flexible if it's the same part of speech?
Take the following examples:
1a. Clean your room, or else I will ground you.
1b. Clean your room, otherwise I will ground you.
2a. Clean your room; I will ground you or else.
2b. Clean your room; I will ground you otherwise.
Sentences 1a and 1b have identical meanings and are both grammatically valid. (I'm not worried about the minor issue that perhaps, stylistically, 1a could definitely go without the comma, whereas 1b might feel wrong without the comma.)
However, while 2b is grammatically valid and has the same meaning as 1b, meanwhile 2a is grammatically invalid without a comma before "or." (We can't put a comma before "or" without changing the meaning of the sentence. "I will ground you, or else," threatens me with punishment if I fail to punish you. That is a very different statement than 1a, which threatens you.) See the definition of "or else".
So, again, my question is, which part(s) of speech are "or else" and "otherwise" in this context? If they're the same part of speech then why does the grammar and meaning change for "or else" but not for "otherwise" in sentence group 2?