You are falling into a linguistic trap. The idea of a 'gene' 150 years ago was a "unit of heritable information". We didn't actually know how that information was stored - but we knew it was there (see Mendel's pea pod experiment). In the last 70 years it was elucidated that the primary heritable material was DNA. And so the older term 'gene' became convoluted with a newer concept of DNA. There is currently no good physical definition of a 'gene' - is it the expressed protein, the region required to express the protein, does it include introns, exons, etc? And so other terminology has been adopted. A 'gene' is not just a strand of coding DNA, and technically can include any 'heritable information' - though the primary and most common concept of a gene is a strand of protein-coding DNA, though most biologists would also include the upstream/downstream regulation of that coding sequence in the definition of the gene (promoter, terminator, introns, exons, etc.).

The methylation of histones (one type of epigenetic modification) which is a chemical modification of a protein bound to DNA is absolutely a heritable modification, and thus is technically a component of your 'genetic information' - but it itself not DNA. Regardless of terminology, a histone's chemical marks (and biochemical/phenotypic effects) are capable of being transmitted between mother and child, as well as between mother cell and daughter cell. If cell A switches from being heat-sensitive with respect to X, to no longer heat-sensitive with respect to X because a particular promoter was silenced by a histone modification, that phenotype will be passed down to the all of cell A's children (including gametes). That is absolutely a heritable genetic characteristic that has nothing to do with the editing or sequence of DNA.

This gets confusing because now our newer definition of gene, which was simplified to be synonymous with coding DNA, actually is more complicated.

Finally, gene expression is not just a developmental topic. Gene expression is not only what determines how you develop, but everything else in your body too. Gene expression is how your stomach determines how much enzymes to produce in a given hour, how your white blood cells determine which antibodies to produce in a given season, how your brain determines which neurotransmiters to produce in a given hour, how your skin determines how much pigment to make, how your beta cells determine how much and when to produce insulin, or how your body keeps track of its circadian rhythm, among everything else your body does. Gene expression is one of the primary mechanisms of bioregulation - especially for biological changes that take between a few hours and a few generations.

I do not understand your discussion of disease models/virus/pathogens. I'm unclear how those concepts are relevant here.

A hypothetical cultural example: A young boy is not well fed and eats a lot of a certain kind of hard-to-digest plant in place of meat. That young boy's own gene regulation may shift towards those enzymes which can break down that plant material and away from genes used to metabolize meat. Further, the entire metabolism may slow because of his cultural environment. It is very reasonable that those regulatory modifications to his own metabolism will be passed to his children. Not changes in the sequence of DNA, but heritable genetic regulation none-the-less. The effects are subtle, and can likely be relatively quickly overcome were the child to experience a significantly different cultural environment.