A wet niche pool light brass conduit needs to be extended using PVC. Is there any violation in the proposed drawing?

I think you have met the intent of the code but your local AHJ might question the method. You do understand the pipe is under water and the joint has to be water tight?

I've never run a grounding wire outside of the conduit in that manner. I'd run the ground wire inside the pipe from the fixture all the way to the box.

If you run the bond wire to the niche, you need a lug for the purpose (may not be there on one made for brass pipe) and it needs to be potted. It would make this a lot closer to the letter of the code. The pool will need to be drained down to cut that pipe anyway. I am not really sure how you would mount a lug in a metal niche. Sheet metal screws are out and it needs to be water tight.

The brass conduit is under the deck for 10 feet then comes up in a planter, so no way to get to the niche without cutting up the deck. The threaded coupling on the 90 coming up broke. It is my customer but since I don't do pool wiring I referred them to the local pool builder. She didn't like his price so she called her pool maintenance guy and this is how he proposes to have it done, but he himself won't do it. (he also suggested a brass plumbing union) I won't do it either so we are at am impasse. Not only does the break need to be repaired but she wants the deck box moved over a few feet to make way for french doors.

Can you find red brass pipe? That would be the best way although I am not sure if you can find a metal J box. I suppose they still have them but I have never seen anything but PVC

I think that brass "conduit" is really plumbing pipe anyway (tapered threads) because it needs to be water tight. If the AHJ is on board I would use a brass coupler and an RNC male adapter. Your bond wire is just taking the equipotential grid to the J box. That grid allows any chapter 3 method of splicing. There should also be a EGC in the lamp cord.