Both caves in question are solid caves, not difficult to find, with good trials leading there, so shouldn't be a problem not taking a tent along - just make sure you have booked the caves before the time. You never know in the Berg, but the likelihood of it being very wet around those dates is lower than in summer proper. And the river-crossing issues are normally only in mid to late summer, like January through to March.
Yes it would be do-able going up and down Leslies in a day from Marble Baths, but still long and not to be underestimated. To get to the base of the pass is time consuming along the boulder bed, unless you can find the overgrown trail next to the river. The pass itself gets pretty steep in places, and is easily underestimated because of its popularity. How fit is your wife and is she familiar with the Berg? It may not be the best idea if she is new to the Berg, and even more so if she is new to hiking. If you attempt it, be very disciplined about a pre-determined cut-off time. No major security issues that I'm aware of, just don't leave valuables behind in the cave.