Some of these wet lenses are manufactured that there will be no vacuum between the port and the lens so even if you screw on the lens on top water "will" enter between the two as there are holes provided. They are also made to specifically have a layer of water so a vacuum will not work. Inon has a wet lens with a dome in front that works with a vacuum between the two. So in such a case water will NOT enter. so depending on the lens is the use.
As for your second query, strobes are your best option for stills. Lights are no substitute. Two such lights will work for stills BUT you will need very large apertures and slow shutter speeds and the results are nothing as compared to strobes. They will work well for video though. There are enough discussions on this on WP so check that out.
I have used the SMC and have got some really good images which are tac sharp. The really difficult part is using this with the Nauticam super multiplier added to the SMC. Here are some images that have been taken with both stacked together. I was quite pleased with the bokeh kind of result. BUT NOT easy to use.