Hi Caliburn!
If you touched all the runes and sticked the staff into the stone, then the mushroom patch should have appeared quite close to the stone, in fact, I think you can directly see it from there. Walk around the ruins in circles (well, just where the trees are), you should definitely find it there...
For the bard: Quite late to change, I changed my at level 19, but it was a good decision, and, well, for the Giant Sword... go ahead, bard instruments have a maximum requirement of level 18, and after that your bard won't develop much, so it is good to let him have a second career. Especially with such a nice tool
Never forget your towel, cheers
Philipp