As for the rest, generally it is a far sounder strategy to open in a non-destructive manner, then test. Testing first opens up race conditions.

Beyond that putting in a ton of paranoid checks tends to create unmanageable messes. The harder you make security, the less likely it is to happen. Make it easy to be secure (eg through a small number of functions like I wrote above) and think about how it fits in your overall policy. (I might work as a non-privileged user in directory structures whose permissions are locked down to just that user, then leave it at that. If I want to put a symlink in there, that is probably OK.)

In general make sure that things are sane, you have programmed in a way where unexpected inputs cannot be misunderstood, and make it simple to maintain that. But if (and without seeing what you do I have no idea whether this applies in your case) you set up a complex scheme that is supposed to be followed, you have set yourself up for failure. Complex schemes tend to erode security.