Crown transplants can certainly be undetectable. However the crown tends to be viewed as a “black hole” for grafts. The convex nature of the surface + the natural lay of the hair direction makes it a much less graft-effective place to try to thicken. Transplants go much farther for your graft/dollar on the top or front of the head.
I would urge you to rethink the hairline issue.
I also have a dim view of the majority of transplanted hairlines on the net. But really, do you know a “natural” hairline from an unnatural one?
I find that good hairlines are like trying to draw a nose or an ear - once you start trying to draw a perfect one on paper, you’ll keep erasing & retrying it forever. But when you just see one of these “imperfect” ones in the real world in the context of daily life, you usually never even question most of them.
And if you posted (unlabeled) pics of 10 natural and 10 transplanted hairlines up on the website, I suspect that at least half the natural ones would get pegged as transplant work with some “flaws.”
I think most HT hairlines fail for avoidable reasons.
#1 - a mediocre hairline + a thick-ass frontal third is more convincing than the best hairline on earth in front of a weak head of hair. I think if the rest of the head is shaky then people will scrutinize the hairline and decide there must be something wrong with it, regardless of whether the hairline itself is the giveaway or not.
#2 - The trick of using thinner-shafted neck hairs on the front of the hairline is WAY under-appreciated IMHO. This idea only became an option after FUE was invented, and I think that factor has slowed the recognition of how beneficial it can really be. FUE has been widely done for most of a decade and this trick still doesn’t seem very commonplace.