1. Yes it is possible to induce clipping in these programs. Some of the presets do not have limiters in a given effect and even the EQ module can induce clipping/distortion if abused. Keeping the main mix feeding into the mastering software down a little will often help. Overpushing the input is the easiest way to get Ozone to clip, more so than pushing the output.
Many of the presets do have plenty of compression/limiting protection, but some do not.

2. It is volume that causes clipping, but how that volume is created can be many ways. As mentioned above, simply boosting a given frequency in the EQ section can unintentionally cause clipping on a certain frequency even if the rest of the frequencies are well below any clipping level..

Listening closely and trusting your ears and your monitors is the solution.

3. dB upper limit is often misleading and there better tools for analyzing the power of a mastered file. Read up on other ways to measure loudness such as RMS, Peak-Average, spectrum analyzers, etc.

Again trusting your ears is a big part. Ears should mean more than meters in the final judgement, but meters can help. It would be common to find a recently mastered piece that is in the -.5 to -.2 dB range. But these too can vary in actual percieved volume.

I do not work here, but the benefits are still awesome
Make your sound your own!