It's not that she doesn't trust Redana, you understand.

For all her faults, Alexa has never seen the princess flub a ritual. She's expert, punctilious, every motion practiced and precise, every syllable enunciated with the exact intonation required. There's no reason she couldn't perform this augury just as well as Alexa.

But... Look. These crabs are organized. Efficient. They've even formed into regiments! You don't get that without the touch of the war god. Which means that going into battle without first ensuring that she favors you would be ludicrous. 

Almost as ludicrous, you might say, as insisting that the ward should come along with her into danger when she could just relax in an area without killer crabs. Yes, she knows that Redana and the hermetician cleared this out--she just about had an aneurysm when she found out! How can she guard her when she insists on actions like this?

No. She will not risk this until she's positive that Athena favors them and not the crabs.

(The letter burns against her chest, secure in its newly sewn pocket. She agonized over that--what if it gets wet down here? Redana insists that she mostly cleared out the water, but the thought that the precious information in the envelope might get [i]damaged![/i]  It's only the even worse thought of leaving it alone, unguarded in her quarters, that saw her looking for a needle and thread.)