Fair enough and no harm done, Flooter ^.^
I feel that assessing and understanding people is one thing, but I don't feel it's what we usually mean when we talk about trusting people. It's definitely important, just as one can say "I know this person's drives and motives, so I know they'd sell me out in a second to pursue their own goals... so I had best not put us in a position where that can happen" - but that doesn't mean you can trust them, just that you can predict them and act accordingly.
Here's a metric to consider: How far out of the pursuit of their own direct goals will this person put themselves in order to look out for me? That is to say, if we are pursuing mutual goals, and can obtain them, but a situation arises where I need help, but those goals might be delayed or their chances of success harmed to do so - to what extent delay or risk will this person wear, to save or help me?
Shadow: Her 'front' of a cold Sharran would demand none at all, while her underlying instinct would want to push greatly to help - I cannot know which way she will flip, however, and so for now I cannot 'trust' her as a companion. If she were not busy being a Sharran, she very much would be the sort who would put herself out or at risk in order to pursue right action, or assist me in a time of need... but her other side is not.
Astarion: Depends on how much he feels like he still needs me, and how funny he thinks my failure or death would be; he would not dash back to pull me up from the cliff while running away from the giant boulder, but he would look back over his shoulder to see what happened. I cannot trust him.
Lae'zel: Will bend her goals and their chances by very little, if at all, and would leave me to fend for myself if helping me risked her own chances. I cannot trust her as a companion.
Wyll: Quite possibly the worst of this bunch; he will make assurances to the contrary, lie cheat and deceive to feed his ego and image, but when the chips are down, he will not actually *be* the hero he sells himself as, if it means delaying or impacting his goals. He is a completely disgusting character, untrustworthy in the extreme, and his continued bare-faced lying even when every involved party knows he is lying only makes this worse. He promises the druids that he will help protect them... but the *Moment* he learns about where Mizora has been taken, his immediate goal is to pursue HER, NOT to continue with removing the goblin leadership and ensuring the safety of the grove - he wants to abandon the druids and go after Mizora *immediately*, and his promise to them be damned. If the same thing happened to me: I needed Wyll's help for something, and he had to choose between keeping the promise that he made to me to do so, or pursuing a hot lead on Mizora... we've literally already *Seen* which way Wyll leaps. Wyll cannot be trusted as a companion, on almost any level.
Gale: Gale is the ONLY companion who, upon seeing a terrible situation and wickedness that the party has the power to right, which has *no bearing* on their actual goals in the moment, is strongly in favour of stepping in to do the right thing. He steps up to motion this before Wyll, and there is no-one around to grant them fame or glory for their actions - not even the good will of the people they save, if they manage to save anyone; it's just the right thing to do, and he is the one who wants to do it. Gale is a pragmatist - a ruthless one, for sure, and a lot of people underestimate how grey a ruthless pragmatist can be. He is not a 'Good' person - his pragmatism prevents it for the most part - but he tries to be, when it's feasible, and he's more or less our only companion who will, after assessing a risk (I fully expect that if he believed that the party could not handle the threat with acceptably low risk, he would advice discretion instead, even if that meant more would suffer), put himself out to pursue right action. I feel that, within moderate limits, I *can* trust Gale to save or help me in a moment when it may impose risks or delays on his personal goals, if he views helping me as the right thing to do. Those moderate limits may not be very high, but they're higher than for any other companion, and his efforts towards right action, when they occur, do not seem to be purely performative - as Wylls most definitely are.