Do you think any of the mutations could have been harmful for the beetles’ survival?

I believe your stance on mutation is in needs to be re-evaluated. The articles I’ve been reading say that mutations are either harmful or neutral. There are, however, such things as beneficial mutations. For example, there are people with mutations in the CCR5 gene that make them virtually immune to HIV. There are also a class of cancer drugs that are 100% effective, but only in a minority of individuals that have a certain mutation. Both of these mutations could seriously improve a person’s chance of survival. I therefore fail to see what makes you think all mutations are bad?

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.