<p>Federated Edge Computing (FEC) has emerged as a critical paradigm for delivering low-latency, high-bandwidth services to mobile users by leveraging resources across multiple administrative domains. However, maintaining stringent Quality of Service (QoS) guarantees in such dynamic, heterogeneous environments presents significant challenges, primarily driven by user mobility, resource constraints, and fluctuating network conditions. This paper provides a comprehensive review of QoS-aware service provisioning in FEC, systematically analyzing the interplay between three core research dimensions: mobility management, context-awareness, and reliability. We critically examine state-of-the-art strategies for service deployment, mobility-aware service migration, trajectory prediction, fault handling, and multi-dimensional context integration. Our analysis reveals a fragmented research landscape, where these critical aspects are often addressed in isolation, leading to significant gaps in integrated frameworks capable of holistic optimization. We identify and synthesize these gaps, highlighting the lack of solutions that simultaneously manage resource-stressed fault scenarios, noisy trajectory data, and multi-objective QoS trade-offs. Finally, the paper outlines a forward-looking research agenda, proposing a conceptual five-layer integrated framework design to guide future research, while emphasizing the need for AI-driven orchestration, blockchain-enabled cross-domain QoS contract execution, digital twin-driven full-process optimization, generative AI integration, 6G-FEC collaborative guarantees, and sustainability-aware provisioning, as well as a critical analysis of practical engineering challenges, to enable adaptive, reliable, and efficient service provisioning in future federated edge environments.</p>