Eikadistes, thank you so much for your detailed, thoughtful post!
Regarding ultimate responsibility impossibilism, one can have proximate control without ultimate control. That is, one can choose to follow the advice in the latter part of my document while understanding that one's choice can be fully expressed as a function of factors (such as one's genes and upbringing) that are entirely outside of one's control.
Regarding moral nihilism, following the advice in the latter part of my document does not require the existence of moral facts. In fact, such advice is based on one's desires and interests, whereas moral facts, if they existed, would be independent of one's desires and interests.