Yes to the ISP providing the fibre service to you. I am not sure if one could stay with Telkom for your fixed line telephone service when using another ISPs fibre termination. It really depends on how OpenServe sort out the line terminations.
A good question to ask I suppose IF one day I get the opportunity to convert from ADSL to fibre.
Uhm. No... You can port your number to *any* VoIP provider - even Telkom. It does not HAVE to be your ISP. Frankly, many ISPs do not even offer voice services.
Depending on how OpenServe installs the fiber the copper is pulled out (used as a draw wire) very often when the fiber is pulled in. Your POTS is then converted to VoIP which is provided by Telkom. Unless you port your number, or get a new number from your VoIP provider, your voice service remains at Telkom - whether it's POTS or VoIP, copper or fiber.
Yes the ONT Telkom provides does have POTS ports (2 I think), and your existing analogue phone can be used in this case. If you port your number to another service provider, you more than likely will need a VoIP phone yes (well worth it in any case).
As for the copper cable... If there's fiber in the area and the copper get's stolen and/or damaged, I am fairly confident Telkom won't bother to replace it. Frankly, even in areas where there aren't fiber, getting Telkom to replace stolen and/or damaged cables is already frequently not happening.