Recently, as part of the effort to enhance fuel efficiency and reduce costs for eco-friendly vehicles, the R-gearless system has been implemented in the TMED (P)HEV system. Due to the removal of the reverse gear, a distinct backward driving method needs to be developed, allowing the Electronic Motor (e-Motor) system to facilitate backward movement in the TMED (P)HEV system. However, the capability of backward driving with the e-Motor is limited because of partial failure in the high-voltage system of an R-gearless system. Thus, we demonstrate that it is possible to improve backward driving problems by applying a new fail-safe strategy. In the event of a high-voltage battery system failure, backward driving can be achieved using the e-Motor with constant voltage control by the Hybrid Starter Generator (HSG), as proposed in this study. The introduction of feed-forward compensation for variable constant voltage control allows for the securement of more active output power within the confines of limited HSG output power. In the case of HSG failure, the risk of overcharging the high-voltage main battery due to the back electromotive force of the HSG exits necessitates the disconnection of the main relay. However, the proposed high-voltage battery protection strategy in this study resolves this issue, enabling the maintenance of the main relay closure for backward driving through the use of the e-Motor. In the event of a control area network failure, a redundancy system permits the passage of e-Motor power, ensuring the availability of backward driving. Furthermore, if backward driving becomes unattainable due to e-Motor failure or similar issues, system fail-safety is upheld by providing the driver with a warning notification and automatically engaging the electric parking brake system.