Every time? No, I'm not suggesting that it is literally the same cave every time. Most of the time? No, I'm not suggesting it is the same cave most of the time. I know it is the same cave. As to the man/dwarf/elf made mine/caves, well there are only so many ways you can dig a cave during a certain time period. Pretty sure mines dug in the 1800's look different from those dug in 2000's, but pretty sure those dug in 1800 look pretty similar to those dug in 1805 or 1810 in the same area.
As to beaches I know for a fact there are at least two different beaches layout in DA2. I also know for a fact Hawke visits one of them more than once on different quest. It makes sense at least to me since you take the same path to reach it every time. I'd be more upset if it were different given I've lived most of my entire life within 10 miles of Surfside beach and it has not changed much in 30 some odd years. Beaches do change after major storms (hurricanes), but the only major storm we heard about in DA2 happened when Hawke was journeying to Kirkwall. Surfside Beach change somewhat after Hurricane Alicia in 1983 and did not really change again until Hurricane Ike in 2008.