The standard way to tell soneone to do something is to tell them to do it "a bit". This "softens" the command, makes it nicer. It's abrupt and even rude without this.
And I believe a native speaker said in another thread in this lesson that if you just say an adjective by itself that's not necessarily understood as a command. So if you want someone to slow down, you can't just say "慢“. You have to say something like "慢一点儿“.
一点儿 or 点儿 can be used to mean "more" or be used like the suffix "-er" after adjectives. So it's saying "slower." http://akikolingoland.com/en/a-little-bit-of-%E4%B8%80%E7%82%B9%E5%84%BF-yidianr-%E4%B8%80%E4%B8%8B-yixia-%E4%B8%80%E4%BC%9A%E5%84%BF-yihuir-%E6%9C%89%E7%82%B9%E5%84%BF-youdianr/
Generally yes, though for the example you chose it would need to be clear from the context what the person you're talking is supposed to do. I imagine that if you were talking to someone who was adjusting the air-conditioning system, or if you were checking to see if some food they cooked was sufficiently warm you could say 熱一點兒
Here are some other cases:
Throwing a ball: 遠一點兒 (a bit furtner)
Negotiating a price: 便宜一點兒 (a bit cheaper)
Selecting a size shoe: 小一點兒 (a bit smaller)
If you feel something wrong, report it, bros!
However, 慢慢的 is a complete attributive because it has the marker "的". Thus it cannot be used for imperative.
The following examples all work:
慢！(Halt! Archaic, but it works too. notice that "adjective" can be used like a verb in Chinese.)
Notice that none of them has the attribute marker "的".
Regarding "in a bit" character combination, do the first characters mean "one+bit/unit/sth"? If so, what does the third character mean? If not, whence the meaning? It is also not really translated here, but more used to imply 'down' in "slow down" - how literally does it actually mean "a bit" or something like that, or is it a more general/vague sense of diminishing the strength of what was said before? Additionally, an alternative translation offered for the "slow" character was 'slower' - how is this possible if the word is a verb? Is it not?
儿 does have an actual meaning (son or daughter) but here and in most cases it simply denotes a Beijing dialect pronunciation which tacks "-er" onto the end of many words. It is a meaningless suffix that is simply phonetic. You can think of it like being able to "spell out" a Boston accent.