Can I use the leftover marinade to baste the meat, or as a sauce component?
No. Since the leftover marinade has been in contact with raw meat, it is unsafe to use as a basting liquid, or as a sauce. However, if the marinade is heated to a boil for several minutes, it may be used. (Not all marinades handle being boiled, however). Take care to use a clean basting brush rather than one with raw meat contamination.