I have a player that wants to be able to use the divine force, but he doesn't want to play a fully trained priest. This made me thinking about adding some options for what he could level up and how to shape the character around.

We found the character advancements rather limiting, especially the low skillnumbers, and this make characters too similar, so I have added two optional advancements per rank. They can be anything. A skill, talent, action card etc.

Also no one in the group wants to buy the extra stances the careers have. They would never use a deeper stances, then 2-3, and I don't want to force them to.

Piety and Invocation shouldn't be a problem if the player use some moderation and respect towards the established faiths. Pious people should be treated with some respect, even though they are acting outside of the clergy. I'm not talking about buying rank3 blessing and 3 levels of training. Rather only aquiring the skills necessary and maybe a few rank 1 and basic blessings.

Magic is more tricky. Casting a spell without the authority of the orders means a trip to the stake, so learning Channeling and Spellcraft is maybe too dangerous to add as a flavour for a character.

In theory he could pick a priest career and just take the skills he wants from it. Once done with that he will have piety etc. and can keep picking action cards with blessings once he changes to another career. Being in the priest career shortly could be explained just as you say. It would give him the added benefit of not using any advancement from a combat focused career on priest skills. The downside of course would be the cost to change career back and forth.

As for the stances. It's the same in my group. They have played over 25 sessions and have only 3 in their best (started at 1 though according to our house rules then). For some attacks however it can be great. Of course the chance to get fatigue rises, but remember even if 6 fatigue symbols show up on the dice, then only one fatigue is gained. Some attacks add damage the deeper you are in the reckless stance.

It all depends on the career and in any case it does take some time to get to 6 reckless. Three rounds if you accept a fatigue each round. So while it does give you great chances for great results, it's limited by time and fatigue.