The point @tfa made was that the code will be forked and was not meaning the data.
Here though forks of the code does not necessarily mean forking the data. And as @tfa says forks will be made for language and other reasons. Especially the UI libraries will be forked if people need differing interfaces.
SAFE is a system of protocols. In essence if you created node code that implemented the protocols correctly and obeyed the protocol rules then the network would accept the node running that code. It is in the protocols that the network is stitched together.
So if your node does not obey the consensus rules, or obey the data retrieval rules, or obey … etc … etc … then that node would be removed from the section and ignored. But if it does its job correctly and obeys the rules then there is no reason for it to be removed. Remember the nodes only see the data/protocols being sent to them and so each node is a black box implementing protocols and rules.
As to running different versions, its like the above, as long as the new versions understands the previous versions of the protocol & rules then all is fine. Just like routers, computers, etc can understand tcp/ip V4 and v6 on the same network, so would the SAFE nodes understand previous versions of the protocol. If they did not then you could not upgrade the actual protocol without a coordinated restart of all the nodes.
Now how many versions of the protocols can be understood is the real question.
Also node upgrades will involve much more than protocol changes/upgrades. Maybe it will be optimisations or changes to the user interface/apis. And I suspect that protocol changes will be rare compared to all the other upgrades.
Now as to how we protect from coercing a developer. Well we cannot really, but what is possible and will definitely happen is any changes will be looked at and pulled apart by a number of people. I had lunch yesterday with one such person who knows their stuff around the SAFE code and they were telling me of another whom they met by chance. The other person asked my friend if he knew of the safe network and this other person told my friend how he had walked through the code. Now I am sure that there are many others around the world like that keeping a very close eye on the code including also @tfa above.
Also one fork I will be implementing, if needed, is one that prevents automatic upgrading of the node’s software. As a System Administrator I do not allow automatic upgrades and only upgrade at a suitable time after I am confident of the upgrade’s viability and security.