Maazito:

Question 1:
First a guess (as I don't use Mac): You can most probably find the libraries that are automatically installed when you look at the subfolder Library/Application Support/eTeks/Sweet Home 3D/ . There probably is a subfolder furniture there. On one Mac delete the contents, restart SH3D and you should have an empty furniture list.

<UPDATE>
There is very relevant info in another thread : it's partially in French, but the two most important parts are in English.
<END UPDATE>

Then I would suggest this: on the other PC install SH3D with all your imported files. Then use Furniture --> Export imported furniture to SH3F...
This will give you a library that you can then import on your other PC.

Question 2:
I think Q1 already answered this one.

Success,

Hans