I may not be able to answer all of your questions but i will try to answer as much as i can...
first off, ALL turtles need UVB lamps, no matter what size or how old they are!
second, you 'can' dechlorinate the water, but i really dont think that is necessary unless you have REALLY HARD water. and even if you have really hard water, the turtle will still live just that it's shell might not look as vibrant as other turtles!
and about the water changer. all i can say is that i know turtles are MUCH dirtier than fish. is it not possible to wash out the changer between the fish and turtle tanks? (i wouldnt know b/c i dont have one)... i hope this helps 
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~ Evie 
Pets:
RIP 0.1.0 German Shepherd {Dusty}
0.1.0 German Shepherd/Collie mix {Shadoe}
0.0.2 RES turtles {Booger & Snot}
1.0.0 African Clawed Frog {Bingo}
0.0.2 Snail {Gary & Larry}