lateral thinking - Find the letters that complete these five patterns

Complete the last line in each of the following five series:

Puzzle 1:

"Using" is E
"Desist" is I

"Visits" is E
"Design" is ??? 

Puzzle 2:

"Asia" is H
"Cuffs" is S
"Ender" is I
"Redness" is F
"Fuchsia" is ???

Puzzle 3 (advanced):

"Pro-tem" and "row" are H
"Ties" is K
"Cohesion" is A
"Casino" is E
"Hocks" is either a word, name, ??? or ???

Puzzle 4:

"Gross" is W or R
"Assign" is S

"Amassing" is E
"Design" is S
"Amassing" is S
"Arrows" is ??? or ???

Puzzle 5:

"Edna" is I
"Slang" is X
"Siege" is E
"Basin" is X

"Sin" is X
"Sin" is A
"Besiege" is X
"Signals" is ??? and ???

First person to solve all five gets the win.

Hint:

Each ??? stands for a single letter of the English alphabet.

Hint #2:

The puzzles are not related to one another, except that they are all based on the same principle. If you solve any one, you will rapidly solve all the others.

Hint #3, a quite cryptic one:

The very first parts of vague solutions can decode nothing: that's the core of these puzzles.

Hint #4, a more obvious one:

You want another hint?! Nag! Go botanist first, arse ulcer! O my! More clues? Rats! Rifts in a toboggan!

Answer

Brilliant puzzle.

Puzzle 1:

U

Puzzle 2:

A

Puzzle 3:

T or P

Puzzle 4:

S or G

Puzzle 5:

I and E

Explanation:

They are palindromes, and every word is part of the palindrome too.

Individual solutions

The beginning, middle and end of the palindrome have been bolded:
using is e desist is i visits is e design is U
asia is h cuffs is s ender is i redness is f fuchsia is A
pro-tem and row are h ties is k cohesion is a casino is e hocks is either a word, name, T or P
gross is w or r assign is s amassing is e design is s amassing is s arrows is S or G
edna is i slang is x siege is e basin is x sin is x sin is a besiege is x si gnals is I and E