Yup. There are a couple of selection tricks worth learning. The two most useful are the rubber band selections - where you drag a box around objects to select them.
Note that dragging top left to bottom right, and the reverse direction give intentionally different results. Down from the top will only include objects that are completely within the selection. If one end of al line is not in the box when you release the mouse button, it will not be included. The reverse direction will include any object where any part of it is included in the rubber banded area.
Once you have the bounding box on the grouped object, select the move tool (push M). when you over over one of the control points on the bounding box the protractor will pop up (easier then needing to explicitly select it). Rotate one axis until it looks right.
That gets the bounding box progressively more aligned with the content as it gets near to alignment with the axis.
Yes.
If you build things, and then move them together - they tend to "clump" and stick together. Which makes further moves and changes harder. By "grouping" something - even if its just a single object like a cube or cylinder it will no longer attach itself to other stuff. You could move it into a position where it passes right through your model, and then move it out again with no difficulty.
In this case I just centred it by eye (hence using an open ended cylinder so I could look down it). One could have used the ruler tool to drag guides away from the existing one for more accuracy. Note also that in a normal model that is not made from a myriad of individual vertices, you could have simply used the offset tool to drag a larger circle boundary from the existing one. The offset tool makes making concentric copies of borders smaller or larger than something already drawn very easy.
The "intersect with" features are harder to explain, but quite handy. They basically allow you to selectively do what happens when you slap together un grouped objects (i.e. when they normally form a vertex at the intersection and become joined there). You can slap a grouped object into your model, then intersect it, and it will add new vertices at all the place they touch. Take away the grouped object after and you now have a complete set of new "cut here" lines on the model.
Pretty much... it was the kind of job that had you have started with someone's sketchup model, would have been fairly easy. The fact that you had acquired the model from another 3D file format gave you are starting point that looked ok (ish) but was always going to be a bugger to tinker with. A bit like trying to change a program when you only have the compiled executable and not the source code.
For the purposes of getting better with sketchup, start with stuff in the 3D warehouse - at least that is sketchup native.
no problem, thanks.