In this paper, we explore the potential, as well as a few limitations, of Cellular - Vehicle-to-Everything (C-V2X) sidelink connectivity (a.k.a., PC5-Mode4) for Cooperative Lane Merging (CLM) services in a specific cross-border highway context. Based on a dedicated simulation flow, we assess both the successful completion of the CLM negotiation procedure from the standpoint of messages delivery (service availability) and the accuracy of position awareness, as perceived by the manoeuvre management application out of collected status messages (service reliability). These indicators are evaluated as a function of road traffic, overall system latency and infrastructure deployment. Simulation results suggest that both Vehicle-to-Infrastructure and Vehicle-to-Vehicle communications based on C-V2X PC5-Mode4, which benefit respectively from information redundancy and short-range links, could advantageously complement Vehicle-to-Network connectivity to locally guarantee the seamless continuity of latency-critical services in challenging environments.