Hi Guy - thanks for posting the file, that makes it a lot easier to answer your questions about it!

For #2 - there is not any hole there, you already have a surface in that spot, here I have selected it to help show it to you more clearly:



So you don't need to do anything at all to fill in that hole - there is no hole there, it's filled in by a planar surface there already.

The only part that actually needs to be filled in is the very top of the piece where you do actually have a hole here:



To fill in that place that really does have a hole, select the whole object by one click on it so that the whole thing is selected like this:



Then run the Construct > Planar command here:



That will fill in an end cap in that planar opening of the object, sealing it off and making it into a solid now:




re: #3 - you actually don't need to worry about that either - it looks like you did a good job constructing your original curves making them tangent to one another and with extrusions if the generator curves are tangent the surfaces that are created are tangent too regardless of whether they are in multiple pieces or not.

So your object is already nice and smooth at that spot, it's ok that there is an edge there, in this case that is just part of the topology of your object and does not automatically mean that it is not smooth, although that could be the case in other situations like if your generator curves were not smooth to begin with or if you had a more complex type of surface there that was flexing and bending in the middle of it and not just an extrusion.

But anyway your object is actually smooth there already, here I've done an polygon mesh export with a high polygon count and shaded mode set, you can see that your existing object is smooth there:



It's normal for extrusions to inherit the segmentation of their generator curves - when a curve is made up of multiple different segments joined together, then when you extrude it the extrusion will be made up of multiple faces joined together with one face for each segment in the original joined curve. That's not an error, that's how it's supposed to work. The different pieces of the extrusion will be smooth to one another if the segments of the generator curves were smooth to one another like yours were.

If you do have just one single curve segment instead of multiple joined pieces, you will definitely have a smooth piece though because you can only have sharp corners in a curve at segment junctures. So sometimes it is convenient to eliminate segmentation in a curve to smooth out any small corners in it. That can be done by turning on control points and deleting the control point where 2 segments meet which will fuse them together into a single segment or also the Rebuild command can be helpful for some kinds of curve repair cases: http://moi3d.com/2.0/docs/moi_command_reference10.htm#rebuild However in your case here you've done a nice job making the curve segments constructed so that they are tangent to one another so there is no problem in having things as segments in this case.

Hope this helps!

- Michael