Questions about Brand Registry — help urgently needed

Looking to start selling my product soon and I’ve got a few questions concerning some Private Label and Brand Registry details. I haven’t ever seen this question being asked so any insight whatsoever would be truly, verily appreciated:

1. Am I allowed to start selling a private labeled product before completing the Brand Registry process at Amazon and the UKIPO?
2. If I were to finish a listing for my product and send my order into Amazon without a label the first time around, would there be any problems if I sent the exact same product, now labeled, under the same listing and using the same FNSKU later down the line? I ask this because I understand that registering a trademark can take a long amount of time, which could push back my predicted timeframe for longer than I am willing to wait.
3. Same question as above: would I get into any trouble for sending my product using standard packaging the first time, and then switching to custom packaging from the second or third order onwards?

I understand that these questions may be sort of low level, but I ask for your understanding. I have looked around and haven’t managed to find any resources that address these concerns specifically. Any and all answers appreciated. Many thanks in advance!