What are some best and worst case scenarios of having transplanted hairs removed from the hairline?

It is very difficult remove transplanted hairs.  Some may have tried laser hair removal or electrolysis.  FUE removal may lead to scarring and worse outcome.

There are a variety of approaches to address a situation like yours, depending on a variety of factors that need to be more thoroughly evaluated.  They include:  FUE punch removal of the lower-most grafts (which in my extensive experience leaves no detectable scarring, and oftentimes requires 2 and even 3 procedures to achieve); a browlift done behind the hairline to elevate the hairline; laser hair removal.I suggest you do your homework to learn all your options.   Your story also emphasizes the importance of choosing a hair surgeon who has extensive experience in hairline design.

I advice you do not have a removal process with FUE because it is impossible to remove all hair shafts from the body area with a surgical procedure that would have possibility of transection of hair shafts during one by one insertions. Plus you may have a chance of scar formation and small irregularities on forehead skin.So laser removal is a best and logical option for thick hair removal .

It is very difficult to fix a poor hairline job once it is done, which is the main reason why it is very important to make sure a skilled hair transplant surgeon is chosen at first place. Once a poor job is done, an even much more experienced surgeon is required to fix it. 
The main treatment is to remove the most unnatural grafts via FUE and transplant hair around the region to improve coverage. Since transplanted grafts are buried in scar tissue, it is more difficult to completely remove them; some are transected which results in regrowth of a part of these grafts. Besides, it is not possible to remove adjacent grafts in order to keep intact skin between punch holes, which is the reason why 2-3 removal sessions may be necessary. The good news is that the removal sites heal very well.

You may be better off using laser hair removal rather than surgical excision.  Your best bet is to meet with a surgeon and be evaluated in person if you want a detailed analysis of your options. 

Removing a hairline because it is too low has its problems. I would have to see the problem before rendering an opinion