(I'll move things forward descriptively. Time to wrap up, I believe.)
From what Wolfred tells you, the cave cuts directly into rock, not dirt, so widening would be difficult. With the branches from the tree, he doubts any sunlight can illuminate the inside. Wolfred thinks that it might be possible to go inside a little and then light a torch. But smoke might be an issue, he says. As he mentions that, he stops before yelling. "There's a slight breeze flowing through, master! So there might be another entrance into this place!"
So, after searching for a little while (using Eyes of the Treacherous Terrain to help), your small group discovers what must have been the primary entrance to the cavern. You are able to use improvised torches to light the way as you explore the place.
Seems like a wizard once lived here, many decades ago. Whatever belongings he had have since deteriorated for the most part. There are a few sealed jars left on stone shelves cut into the cavern's walls. More jars certainly were present, but now lie broken as their content fermented and burst out. No sign of the former occupant's body are found.
Not all of the jars contain liquid, however. Some contain animal bones and teeth, other dried roots and fruits. For the most part, these have decayed so much as to be useless. Herman is still able to identify and recover 2 pawns of Animal vis (a tusk almost as long as his arm worth 1 pawn; a small skull worth another pawn) and 1 pawn of terram (a small river rock of reddish striated stone, that fits in the palm of his hand). There are also some writing materials rolled up in some of the jars, but they are in a language you do not understand. Only two jars seem to contain liquid, sitting precariously on a broken shelf beside the opening formed by the crevice.
(Overall, I rolled 20 stress dice to see whether any botches happened for the various spells Herman might have used to investigate the items found. No botches.)
When the party finally returns to Fengheld, they find the vineyards in a panic. The hundreds of birds that you saw emerging from the tree apparently attacked the vines and damaged them badly, eating much of the grape in a wide swathe. Peter of Verditius is furious with Herman, saying that he was supposed to fix the problem, not make it worse. It is a close call, but Herman is allowed to remain for the agreed-upon time to study from the library, but that time is strained and it is clear that prolonging his stay would be met with refusal.
It just so happens that, in the winter, one of the visiting redcaps approach Herman to discuss what his plans are for the future. Fengheld being a Mercer House, many redcaps visit it so that is not unusual to meet new faces there. What is unusual is the redcap himself -- a nervous man of middle age, with a round face and a thin light brown beard. He stumbles and hesitates much as he presents a proposal to Herman. He knows a small group of magi that would welcome additional magi. They live in an out of the way place, not an established covenant, so conditions are still basic. But the place would offer some freedom from the strictures of older covenants.
If he's interested, the redcap (whose name is Bernhard) could arrange a meeting with them in Anvers at the end of next Spring.
(OOC: You might want to summarize Herman's activities during the seasons spent in Fengheld. It will make it easier to determine how much xp to grant for each season.)