Im sorry to hear about your diagnosis. Fortunately COPD will not interfere with Sodium Nitrite based suicide, if anything it will contribute to methemoglobinemia, which is the root cause of death from SN.
Both COPD and SN impair oxygen transport and delivery.
Im also sorry to hear about your diagnosis. Fortunately for you as well, SN suicide will not be impaired by lupus nephritis. The kidneys don't play a role in oxygen transport. One thing to keep in mind is to be sure you have your dose strong enough as sodium will further damage your kidneys if your attempt is not successful.