Tradução - Português brasileiro-Latim - Que minha vida seja guiada pela fé e ...

Why "potestne"? (-ne only for a question after 2nd word in sentence; here, a wish, subjunctive is required, with (optional) "utinam" in order to insist.)
Besides, here "ab" isn't necessary (fides and caritas are inanimate) unless you want to personify them.

Isn't it a question? English version looks like it is...

This indeed is a wish (attainable) = "un souhait réalisable". I understand your perplexity: its cause is probably that there is an inversion in the English sentence: "may my life be..." But if it were a question, there were "is my life guided..." or "can my life be..." and a question-mark. The original Portuguese sentence begins with a "Que" and the verb is in subjunctive mood "seja" (It. "sia", Fr. "soit", these indicate it's a wish like in a prayer. In French, both of these constructions can be used: Que ma vie soit guidée par ma foi et mon amour en Jésus-Christ!
Puisse ma vie être guidée, etc.
In a way (I'm not sure of it) MAY stands here in English for subjunctive. Perhaps in old times, it could have been : Be my life guided...
How would this be expressed in Polish (subjunctive?)?

Last thing: you've put "posset" in imperfect subjunctive. It implies that wish is unattainable (hence a regret). It needs present subjunctive to indicate life *can* be guided..., despise the "peccatum originale"! Otherwise we're in heresy...

I do not know Portuguese, so thanks for checking it. "How would this be expressed in Polish?" In Polish language there isn't subjunctve. There is only imperative. In this sentence we should use imperative for 3rd person (special form) with "niech". Although it is imperative it expresses a wish, not an order. It would be "Niech moje życie będzie prowadzone". I really like discussing with you grammatical issues:-)