Ok this could very well be dangerous.

If Powys is strongenough to ambush us, and their army managed to survive, mostly, then why do they need our troops?
Either it could be a trap or they are bluffing. I estimate the last. We could likely survive the trek home without much trouble. But!

Since they are bluffing they are also sincere. They would prefer us as friends.
Thus I propose that they get not a single soldier until we are out... Meaning we have the entire army with us.
The prince will travel under our protection with whatever bodyguards he has until that point (hostage, but strong enough to help in case of Gwynedd attacks).
At that time we hand over enough troops to help him subdue Mathrafal, I suspect 150-200 will be more than enough. That should firmly set him on the throne in the view of the troops and people.
When that is done we should be home taking stock of the matters at home and in a better position to see if we can help with a few more troops to gain an ally/puppet (we should demand that for the help), or rather leave Wales in an almost perfect unbalance where neither side is strong enough to overwhelm the other within a good time.
Food they can get when we get out, it is not a major matter out there, in here it is. The food we have will last us out of Wales, it is doubtful Powys can/will provide enough for us when they demand food in the first place.

We must remember we are the strong party here, so we are the ones to make the demands, and I think my demands are fair enough to get a positive response.

Should it be forcibly rejected we could still take him hostage. And while in our custody be treated nicely, shown that we had no intentions of tricking him and at the end still hold our deal of the bargain open. The time it would take to get out would likely see him calm down a bit and be more open to our suggestion.