Ryan, I'm a newbie myself, but I have an A720IS and can help with some of your questions:
1 - no lens attachments on the OEM housing.
2 - given a tray and arm(s), you can mount a variety of external strobes, but they must be fiber-optically triggered (sensing the onboard strobe's activity), and they must be able to ignore or mimic a pre-flash (one flash) in the onboard strobe, unless you want to shoot entirely in manual mode with the pre-flash shut off (which may be the best way to go anyway). BTW, I gather that some extra shielding of the onboard strobe may be necessary for some units, as there will be light leaks otherwise which tend to throw off the external strobe - I don't have any direct experience with this yet myself.
3 - yes, you can use the zoom with the WP-DC16, but the odds are good that you won't want to much underwater - remember the relationship between increased focal length and the need for more light (which is hard to get from an UW strobe), and note that this relationship is even more pronounced with P&S cameras, because they have small CCD chips and record less light than DSLRs.
I chose the 720IS because the difference in price was so small that the camera's extra capacity on land seemed worth it to me, but YMMV.
Incidentally, I got two very good pieces of advice here and on other boards that I'll pass on to you. First, I drowned my camera by dropping it in the water off a dock before I ever got it into the housing, and had to replace it (the photos I lost were, of course, irreplaceable)
I was advised to put the replacement in the housing right away, both to protect it and to get familiar with using the controls.
The second was to consider getting one or more of
these filters, which can obviate the need for strobes for some kinds of relatively shallow water snapshot photography. They will also allow you to use the camera's DV capacity, which burns up memory, but is an interesting feature (strobes are useless for this type of use, of course). I haven't tried this yet either, but the results look remarkable, and they came well recommended, so I'm looking forward to it (they're also cheap enough that an experiment is worth it, IMO, and they can be cut to fit inside the housing without sticking them in, so they can go in and out at will).
HTH,
- John