Hi Peter,
The collapsible model helps with portability, especially if you have a small vehicle or you travel to dark sites. I generally find I leave my solid tubes at home alot rather than take them to the country. If you are observing mainly from home,then its less of an issue. In regards to focus if you have the collapsible model, people can adjust the struts accordingly to achieve focus.
Not sure about the fine tuning ring. Maybe someelse can help out here.
Yep in regards to the focal length of the eyepiece. You need to divide the focal length fo the eyepiece with the focal length of the scope.
The 10inch you are looking at is 1200mm , so the eyepiece you selected will yield X240 magnification - most nights you will limited by the atmosphere.
I did do a compassion with the solid tube, collapsible and goto. - I found goto useful in find objects, taking short videos/shots and outreach. Obviously the cost is much more but I found them useful for me.
I did find that the collapsible and goto have plossl eyepieces, verses the solid tubes super. -To be honest I would not pick the scopes based on the eyepieces the scopes come with. The standard plossls are ok , but you will probably end up getting your own ones later
Hopefully this the info helps.
PM or post and I will try to answer as best i can.
m11