Bringing him to the edge made him a martyr and an international celebrity. Bad move.
Killing him would make him an even greater martyr. Or it was a failure. It can happen to the best. In 1997 there was a failed assassination attempt by Mossad against Khaled Mashal. Mossad agents managed to poison him, but they got captured and as result Israel had to exchange the antidote for Mashal, who was in a coma, like Navalny, for their agents. The antidote was brought personally by the head of Mossad, Danny Yatom, to Jordan.
If he dies in prison for any reason, the whole world will know and scream.
And will probably forget/move on (especially that, of course, there will be a "sensible" health-related "explanation"), just like it was the case with Navalny's "predecessor" - Boris Nemtsov, who was murdered in 2015 close to the Kremlin walls less than two days before he was due to take part in a peace rally against Russian involvement in the war in Ukraine and the financial crisis in Russia.
and an offer he would eagerly accept.
Navalny doesn't strike me as someone who could be bought or threatened, tbh...