You can use the same valves and springs in the 1.6 as the 2.0 so yes all okay to use.
Better off keeping the 2.0l camshafts in though so you get a little more power.
Changing the bottom end bearings require you to remove the sump.
Yes you can do them in situ.
If you don't and your engine has suffered a snapped cambelt it will work for a few thousand miles before you'll get bottom end knock.
This is due to the impact of the piston hitting the valves causing a flat spot on the bearings on the bottom end. Eventually this gap wears further and further till it gets sloppy.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts