In the sentence below, is くれ imperative form of くれる?
No. It's the negative-form. Imperative-forms usually end the sentence. To understand this I'll break the sentence down:
Let's remove the parts
なくては困る to understand what it is.
From here we'll add words back to the original. Let's add
ない. We have to use the negative-form, which happens to be the same as its imperative-form^.
ては is actually 2 words. Conjunctive particle
て + binding particle
は. It indicates an assumption that leads to something bad. Dictionary. To add the word we have to use the continuous form of ない, which is なく. Now we should be safe to return to the original sentence.
くれる is categorized in 下一段活用, but it's odd one out for its imperative-form. Wiktionary article, search inside the page for