English latin translation

I believe Defende and Protege are correct.

I have never run across Assiste. Where did you get that?

I would use Adiuve rather than Adivve.

Someone told you that in Classical Latin V was used for both U and V.
That's true, but in Classical Latin, there were no lower-case letters either.
In the Latin that is used for mottoes and such you will see a mixture of
upper- and lower-case letters and the use of the letter u.

Question. What's the difference in their use, like in the word "Adiuve?" Do they represent different sounds?

In the "Classical" pronunciation, I believe the word would be pronounced
Ahd-ee-OO-way
I learned my Latin in an ecclesiastical setting and I would pronounce it
Ahd-YOO-vay

Here's a good summary of Classical pronunciation.