Hi Guy, yes like Pilou has already been discussing with you for your #2 instead of using "Trim" which is a surface cutting operation you most likely want to use Boolean difference which is a volume cutting operation and is focused on generating solids as the output.

When you cut an solid using boolean difference, the "side walls" of the cutting piece will be incorporated into the result, making the result a solid as well.

Trim is more of a surface modeling operation that only cuts the outer skin of the object - it's a more manual low level tool that you can use if you are working with surfaces instead of solids.

If you are working with solids, it's more convenient to use the boolean operations instead, basically the booleans do a combination of the world of Trim + deciding which pieces to discard automatically based on which volume they are in + Join the pieces back together again. You can get the same result by doing a bunch of those operations yourself manually but it will be a bunch more work.

Here are some other previous examples showing how you want to use booleans to cut your object if you want "side walls" of the cutting object to remain:

http://moi3d.com/forum/index.php?webtag=MOI&msg=5075.3
http://moi3d.com/forum/index.php?webtag=MOI&msg=3883.3

- Michael