You ask some unclear question. Specifically, you state something (“XCell.getValue() always returns [0.0], for formulae of date type, of text (string) type, etc”), without some example data + code; then you list some Q&As that look like should had answered you…
E.g., you could provide a sample data (like “A1: foo; A2: bar; B1: formula1; B2: formula2”), and the code that does not work for you. Then we could try to see why e.g. the first link you posted (with my answer telling how to obtain string as formula result) doesn’t work for you; and why “date type” gives headaches when it shouldn’t.