You have 4 Shamans. For Arena you will not play 5. Its not viable. You will have a healer.
Keep your 4 Shamans, let your friend Tank for you when you want to do instances.
Add a Mage as your 5th. This way you always have ports and food/water for PvP. When you PvP/PvE you will only play 4, but that 5th is always there. If you really really want to be self dependant, roll your own Pally Tank.
There are the answers to all your questions.
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