I have some questions regarding fibre as I am moving to Pretoria next year to study, and fibre may be available where I stay.
You may find
this thread helpful.
As to your specific questions, here's my take (as a WebAfrica/Telkom FTTH client) :
Are there any offers for only fibre cable rental (no data)?
"Naked FTTH" (ie line only) isn't being offered openly (yet). Most providers (MWeb, WebAfrica, Vox etc) bundle fibre line with data. However WebAfrica do allow you to contract with them for line only, albeit at a R100pm premium on the basic line rental. You could also buy the smallest data package available with them, and then contract with another data provider of your choice for additional data. I think the other providers will probably follow suit, if they haven't already.
In the longer term, I think we can expect to see Telkom offering FTTH "line only" in the same way that they do xDSL.
Can you use DSL data on a fibre optic cable using PPPoE? (I want to share our capped DSL data with my parents so it's cheaper)
Generally, yes.
Once you are connected to one of the Telkom access networks (by ADSL, VDSL, FTTH etc), the connection from there to your own ISP goes via Telkom's IPConnect system. All of the major SA ISPs use IPC to be able to provide services to clients via Telkom's access networks. The IPC connection to your ISP looks the same irrespective of how you are connecting (fibre or DSL), and after that you are basically just "buying data" from the ISP. On WebAfrica, for instance, the account management console looks identical whether you connect with FTTH or xDSL.
You can also have multiple accounts (PPPoE logins) with multiple providers, however the endpoint device that Telkom provides (the ONT - Optical Network Terminal) can only connect to one account at a time.
Is fibre installed professionally or just left there for you to sort out? (like they run it across your lawn from the box, put it through a wall, and attach it to a router vs. running it along the walls and glueing it in place)
It's a bit of both. You are responsible for providing a "route" from the nearest FTTH "access point" to the point where you want the terminal equipment. The type of access point differs in different parts of the country : sometimes it is an aerial junction box on a pole in the corner of your yard (piggybacked onto overhead voice/DSL lines) and sometimes it is a cabinet on the pavement. I believe the former is more common in Pretoria (certainly true in my area).
Generally the Telkom installation teams will string an aerial drop cable into your home in a fairly crude way. If you want to (or have to) provide a dedicated route by means of ducting, then that ducting is your responsibility. Have a look at the thread linked above for physical examples and pics.
Are there cancellation costs?
The early FTTH adopters have been required to sign long-term contracts (typically 24 months), which have cancellation costs, yes. This is especially true if you are given a "free" router or other freebie devices (like a DStv decoder). There seems to be a rapid move away from contracts to month-to-month agreements, though. The specific contract terms will depend on who you contract with.
