My impression was that, while he was open to testing Palpatine - and if those tests happened to have killed him, then so be it - he wasn't really interested in doing away with Palpatine. More interested in having a partner with which to rule. Granted, he may have had the idea to kill off Palpatine later down the line, once he's firmly established as co-chancellor, but since he's killed before that happens, we can't really know.

I was thinking just of the final showdown. Plagueis dropped his guard and allowed his apprentice to get him drunk and sleepy. Sidious attacked him with Force lightning and a Force choke, and Plagueis was unable to resist and take him out. Plagueis lost that fight, even when he tried using the Force to preserve his midichlorians.

I see. Okay, what about in a fair fight - as in, when he wasn't drunk or asleep?

On another note, and not entirely fitting with this thread, so I'll be brief: The back cover blurb asks if Plagueis is the only Sith to have never died. Do you think that's a ploy to make the reader think, as the novel continues up to and including events of TPM, that Plagueis was never killed? As a way of lending intrigue to the plot, where we'd otherwise have thought that he wasn't actually killed. The prologue could add more confusion and intrigue if this was the case. Or do you think it's hinting towards Plagueis living on after death, and that his attempt to use the Force to preserve his midichlorians succeeded, and that Palpatine didn't sense it?

Was Plagueis killed because he couldn't resist Palpatine's onslaught or was it because he was so convinced in his abilities to command the midi-chlorians that he actually thought he could survive.
It seems in the novel when Plagueis is attacked he has a moment where he could have chosen to counter attack or at least try to block Sidious' attack. Instead he seems to immediately start to manipulate his own midi-chlorians.

He was stupefied by yet another political speech and couldn't react quickly enough to save himself.

He couldn't heal himself enough to get rid of his raspirator, why would he asume, that he can now insta heal a Force choke.

Because he doesn't heal himself, but he commands his midi-chlorians to keep him alive, there's a big difference. This is how he survived the attack on Coruscant in the first place. The respirator is, I think, just so that Plagueis doesn't have to continually force his midi-chlorians to keep him alive. But after his throat is slit by those throwing knives he should have been dead, but he willed his midi-chlorians to keep him alive.
It seemed to me when Sidous attacked him he was confident that he could overcome anything Sidious would throw at him and only realized he was wrong when it was to late.

He was drunk and weary, blasted by a Sith lightning and than choked, while listening to "this is Plagueis in the form of Hego Damask...." speech.

Sure he tried to influece his midi-chlorians, but it was all he had left. Sidious even mocked him for it.