Some interesting questions.
It is quite possible to complete the flight under IF rules right to touchdown, provided that you are in VMC where there is no letdown available. Obviously weather criteria must be taken into account when it comes to working out ones minima. A IF flight plan does not have to be cancelled on arrival, provided you are able to proceed and remian VMC when below MSA.
As to the departure: You can depart IFR provided you are able to remain VMC until you have reached MSA, (or are able to proceed to your take-off alternate without the safety of your flight being compromised)
As to going IFR to an unmanned airfield, and diverting to another one, your main criteria would be weather related, hence you would have to ensure that the forecast weather is equal or better than the MSA, as you would have to be VMC to go below the MSA as there is no letdown avail to you. (The various rules pertaining to use of take off and landing alternates is avail in the SA AIP )
A visual approach does not cancel your IFR flight. It is an extension of the instrument aproach, however ATC will separate you from other traffic. A very interesting point which many pilots do not know is that you are REQUIRED to remain in controlled a/space when doing the visual approach, hence terrain criteria will not come into account, as controlled airspace has to be constructed to ensure terraian does not appear (for want of better words) in it. (That is why the lower limit of controlled airspace is always a VFR level and is always 700ft above the highest obstacle within 5nm. Hence if you are IFR and fly at the lowest IFR level of controlled airspace you will always be at 1200ft above the highest terrain within 5nm of your position.
As to the missed approach on a visual approach: There is not one - when you think about it, you should be landing assured when you execute the visual approach, hence there should be no need for a missed approach (otherwise you would not request a visual approach, or at least you should not!). If you have to go around, due to whatever, you will communicate this to the tower, who will give you a missed approach to follow. If the aerodrome controller requires you to go around, they will tell you what to do. What if you have a radio failure: Follow the published procedure which says, continue in VMC and land at the nearest airfield - the one you are going to..!
Your can file IFR to an unmanned airfield at any time and carry on righ to touchdown, obviously you will have to do the approach in and land in VMC.
Hope this clarifies it .......