Also, assuming you assign utility to lifetime as a function of life quality in such a way that for any constant quality longer life has strictly higher (or lower) utility than shorter life, then either you can't assign any utility to actually infinite immortality, or you can't differentiate between higher-quality and lower-quality immortality, or you can't represent utility as a real number.

Could you explain that? Representing the quality of each day of your life with a real number from a bounded range, and adding them up with exponential discounting to get your utility, seems to meet all those criteria.