The way Lady Felicia presented her case, with truths and lies so seamlessly woven together that it made it very hard for Alexander to discern which of the things she said were truths and which were lies, although most of the things were the latter.
Lady Felicia had cleverly positioned herself so well that it was she who held all the cards and Alexander being an outsider had little way of verifying most of the things.
The only thing that was easily verifiable were the 10,000 reinforcements and she would be very happy to let Alexander do that.
And even if Alexander could not verify that, even if the 10,000 men did not come, Lady Felicia had already covered her bases for that by alluding to the fact that their arrival was not guaranteed.
While the poison and the schemes were all he said, she said.
After all, it was not like he could simply go ask if LL really wanted to poison or whether it was all a plot by Lady Felicia.