なさいませ and ください has nothing to do with each other. They are different expressions. ください is indeed an imperative form of くださる, which is an honorific verb of くれる, but ください is just a polite request, and can't be said an honorific.
On the other hand, なさいませ is an imperative form of なさいます, which is the polite form of なさる, but なさいませ is rarely used in real conversations except set phrases such like お帰りなさいませ or お休みなさいませ. It sounds like words from a servant/maid/steward/butler to their master.
This nuance of なさいませ is actually from ませ. In fact, just なさい without ませ is an order, and can't be said an honorific as same as ください. Furthermore, くださいませ has a similar nuance to なさいませ.
In conclusion;
助ける
お助けなさる (honorific)
お助けなさいます (honorific; polite form of お助けなさる)
お助けになる (honorific; more common than お助けなさる)
助けてくれる
助けてくださる (honorific)
お助けくださる (honorific; more polite than 助けてくださる)
助けてくださいます/お助けくださいます (honorific; polite form of くださる)
助けてくれ (request)
助けてください (polite request)
お助けください (more polite request than 助けてください)
助けてくださいませ/お助けくださいませ (very polite request, mostly used in business scene or written language)
助けろ (order)
助けなさい/お助けなさい (strong/overbearing order; imperative of 助けなさる/お助けなさる)
お助けなさいませ (sounds like words from a servant/maid/steward/butler to their master, rarely used in real conversations)
助けてください/お助けください is mostly used when the one who wants to be helped is the speaker or the speaker's in-group member(s) since it's from ~てくれる "to do for me". On the other hand, the speaker can't be the object in お助けなさいませ, thus, it can't mean "help me". It's an advice rather than request. This is the main difference in meaning/usage between these two expressions.