Mobile edge computing (MEC) facilitates computation offloading to edge server and task processing via device-to-device (D2D) collaboration. Existing works mainly focus on centralized network-assisted offloading solutions, which are unscalable to collaborations among massive users. In this paper, we propose a joint framework of decentralized D2D collaboration and task offloading for MEC systems with large populations. Specifically, we utilize the power of two choices for D2D collaboration, which enables users to assist each other in a decentralized manner. Due to short-range D2D communication and user movements, we formulate a mean field model on a finite-degree and dynamic graph to analyze the collaboration state evolution. We derive the existence, uniqueness and convergence of the state stationary point to provide a tractable collaboration performance. Complementing this D2D collaboration, we further build a Stackelberg game to model users’ task offloading, where the provider, managing many servers, is the leader to determine service prices, while users are followers to make offloading decisions. By embedding Stackelberg game into Lyapunov optimization, we develop an online offloading and pricing scheme, which can optimize servers’ service utility or fairness, and users’ system cost simultaneously. Extensive evaluations show that D2D collaboration can mitigate users’ workloads by 73.8% and fair pricing can promote servers’ utility fairness by 15.87%.