Would it be offensive to my florist if i bring my own Diyed flowers?

I don't think the worry is offensive or not offensive, but since this is a business not a personal relationship, it's about whether this is allowed per your contract. Read your contract and then ask to make certain.

Check your contract; your florist may prohibit this as they don't want your DIY work to be interpreted as their own.

Good that you're telling them! I don't know that I would add anything to an elevated arrangement below, that sort of defeats the purpose of having an elevated arrangement in my opinion. Unless you're maybe doing some basic greenery at the bottom.

With your contract not specifying, I would just give her a heads up that you have some small pieces you'll be doing to accent. I have a hard time seeing them have a problem with it, as after the flowers are left there's nothing they can do to prevent your from adjusting/altering things around them.

I would definitely speak with your floral designer as she may be wondering what you are doing with the remaining tables. Perhaps she can work with your same budget, change flower options and create both elevated and low so that the flower choice, floral quality, and vision match throughout your space. Good luck.