How much editing is acceptable?

As you should already know, being a seasoned contributor, photos must be top quality to be accepted. Noise being one of the major factors for rejections. 
I too use LR denoise capabilities but, I find Photography AI from Topaz Labs to be much better. Here your photo after slightly increasing the exposure and processing in Photography AI:

I think it's only fair to mention that you're competing with 23 MILLION similar assets. There's a boat load of competition in flowers. So don't be surprised if Stock rejects it for "similar content."  
https://stock.adobe.com/ca/search?k=purple+flowers

Everyone & their cousin submits sunsets & flowers. But few people submit cardboard boxes & takeout food containers. Just something to consider when choosing subjects for Stock.  😉

Good luck.

Make whatever corrections are appropriate to ensure a commercial quality image.

• Proper cropping/framing to create a pleasing composition.
• Neutral white balance.
• Balanced exposure levels (see Histogram panel)
• Sharp details.
• Even color saturation.
• Removal of unwanted artifacts. Etc...

Almost every digital image needs some adjustment in post.

Keep your original RAW images intact in case you need to revert.

Digital noise is probably one of the most frequent reasons for a "quality" rejection by the Adobe Moderators; therefore, it is definitely recommended to use whatever denoise tools you have available. As you've probably discovered, you can overdo it, which causes overall softening of the image. The noise is noticeably improved in your image, however, it's also underexposed, so more editing is required prior to submission.