On-body computing systems offer new forms of interaction, but while they are increasingly integrated into everyday contexts, their unique privacy and safety challenges remain understudied. This paper examines these challenges through a two-round interview study with N = 15 experts in human-computer interaction, and privacy and safety, using speculative scenarios and adversarial roleplaying to elicit insights. Our findings reveal risks specific to on-body interactions, including over-collection of sensitive data, unwanted inferences, harm to bystanders, and threats to bodily autonomy and psychological well-being. Importantly, in the on-body context, privacy and safety concerns are deeply interconnected and cannot be addressed in isolation. We contribute an empirically grounded characterization of these entangled challenges and derive eight actionable design guidelines to support safer, more privacy-aware, on-body systems. This work informs future research and design in ubiquitous computing by highlighting the need for proactive and integrated approaches to privacy and safety in trustworthy on-body computing.