Snipper's page

How do they interact?

For example, I have a flintlock musket with reinforced stock, and stock have shifting rune.
Reinforced stock is have the same number of hands as base weapon, so I can turn it into falchion. And what will happen afterwards?
I can imagine three possible outcomes:
1) There will be musket without stock and falchion and I need to drop something on ground to properly use another weapon
2) There will be musket without stock and falchion, but I somehow can use both simultaneously
3) There will be only falchion, as musket fused with it, and I need to shift falchion back to stock to shoot my musket again.

For me most RAW is first outcome, but I'm don't sure.

There are some feats, that triggers when you roll a success, like Magical trickster or The harder they fall.
And there are some ways to get success on this tasks witout rolling, like Improved knockdown , which gives you auto-crit-success on trip, or Eldritch archer, which gives you auto-success on spell attack roll
So, my question is: will auto-success from feats triggers other feats, or you need fairly roll dice?