They told me the same but they replaced my 3090 Gaming OC for a new unit and I did not have any problem since then (18 days ago). What you have is extremely similar to the issues I was suffering and it definitely was the GPU. Are they going to replace your card?
Regarding the extension bar question, it should be fine with or without it.
I only received the email at 19:13 so they said they would be intouch , I only ordered it on the 10/2/2021 so its was with me 16 days hopefully its still under the 28 days so I can get it swapped.