For example, if one man does not commit idol worship, or eat chametz on Pesach, but attempts to get another man to violate these mitzvot. Would an ancient Beit Din find this man guilty of a capital offense (in the first case)? Would he be worthy of Karet (latter case)?
The question which I decided to answer (this is how I interpreted the intention of its author), is if a global ban, called "...nor put a stumbling-block before the blind..." (Leviticus 19, 14)is considered as if the 'placer' makes this himself (indifferently that its action had the effect that the other fault or not).
It is a question about "ולפני עיוור לא תתן מכשול" = "nor put a stumbling-block before the blind".
See sefer Hachinuch Mistva number 232 The Chinukh says that it has no action, so we do not fight for the offense, there is no mention of Kareth. (If the lake of action is the only explanation, then, the Tana Rabbi Yehuda should punish by 'flogging').
The problem is to make possible a transgression as a bad thing, there is no relation to the nature of the transgression that was permitted.
There are some bans that are prohibited in both directions (to do or to let the other do) as se same action as "lo takifu" (the prohibition of cutting hair in the temporal area) see Makoth 20B
תני תנא קמיה דרב חסדא אחד המקיף ואחד הניקף לוקה אמר ליה מאן דאכיל תמרי בארבילא לקי דאמר לך מני רבי יהודה היא דאמר לאו שאין בו מעשה לוקין עליו רבא אומר במקיף לעצמו ודברי הכל רב אשי אומר במסייע ודברי הכל
May be that the 'client' remains in crouching position to help the hairdresser and so he participates. But it is not the rule in generalBut lifnei Iver is different. MOREOVER, here, "lifne Yver" not need that the facilitated fault has been committed.
No execution, nor Kareth .
One question. It seems that this Lav is lav shebiklaluth (comprehensive ban) and according to all the opinions Ein Lokin Alav, see Ramban on Shoresh 9. Attempt of response: Perhaps there is 2 reasons for the lake of malkuth. 1.- Ein bo maasse (a ban without physic action), 2.- lav shebiklaluth (comprehensive ban).