Hexalobular has covered the other questions well (I, also, don't remember if spawning a new dragon destroys the exit portal – been quite awhile. )
2) How to build a bridge to these islands from the central Ender island (with Dragon)
There are automated systems (ie flying machines that push armor stands with FrostWalker boots [check if this still works in your version, last used in 1.11] or flying cobble generators) try searching youtube for "redstone bridge builder" w/o quotes if interested in this method).
Otherwise it's the basic 's'/rightclick repeat while sneaking… although there is an intermediate where one uses a piston to push out blocks (the pusher station must, however, be rebuilt ever 10 or eleven blocks and at least some of the pistons will be lost to the void rather than recovered).
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.