Going straight to the Torres themselves will usually be crowded (depending on the time of the day). But some of the other hikes less so. I've done the W Circuit (a multi-day trek) and during some days I barely saw another hiker.
I visited Torres de Paine and it was refreshingly different from national parks in the US. On the upside, you can get water and basic snacks at the refugios which reduce the load you have to carry, and makes for an overall safer experience than unsupported wilderness backpacking but still with minimal impact on nature. On the other hand I did not like that they close a lot of viewpoints long before sunset.
Yes, but some tourists change the nature by leaving their garbage etc.
"Nevertheless, recent paleoenvironmental studies performed within the Park indicate that fires have been frequent phenomena at least during the last 12,800 years."
So fires are a normal thing there, or they have tourists since 12,800 years ..