Wireless relay and network coding are two critical techniques to improve the reliability and increase the throughput for wireless communication systems. The performance analysis and the related optimized design of Complex Field Network Coding (CFNC) scheme in non-orthogonal adaptive Multiple Access Relay Channel (MARC) with Selective Decode-and-Forward (SDF) relaying strategy are presented in this paper. Firstly, the upper bound of the symbol error probability, the diversity order and the coding gains are derived for the CFNC scheme in non-orthogonal adaptive MARC. Then the coding gain is utilized as the optimized criterion to determine the optimal power allocation ratio. Our analysis unveils that, for the CFNC scheme in the context of non-orthogonal MARC, the use of SDF strategy at relay can guarantee the full diversity performance, and achieve better reliability with simpler implementation complexity than the link-adaptive regeneration at the relay. Moreover, the optimized power allocation between all source nodes and the relay may effectively increase the realized coding gain, thus further improving the system reliability.