We have already established that lions and tigers are almost indistinguishable in terms of bones etc in the modern age.
Not really, you claimed this was the case but you didn't provide any evidence to support that claim. In contrast
here is a blog post showing several differences between lion and tiger skulls.
However, if you were to discover certain bones of a chimpanzee they might be indistinguishable from a small child or an adult human. Finding a common ancestor therefore would be a fruitless task.
This makes no sense. The fact that a partial incomplete specimen can be misclassified doesn't provide the least bit of justification for the claim that looking for remains that resemble a plausible common ancestor would be fruitless. After all, we don't want to find partial inconclusive remains.
As evidenced by Nebraska man, Peking man, Piltdown man etc.
I don't see how those examples evidence any such thing. 1 hoax 1 misidentified tooth and 1 hominid fossil don't seem to suggest that looking for a species resembling a plausible common ancestor is fruitless.
TTFN,
WK