Forbiddance has the limitation “Range: touch”, which means the caster has to touch the area to be affected. While technically there is no inherent requirement the affected floor space be a contiguous region, composed of 5′ squares or otherwise not possessed of weird dimensions effectively increasing its affected area, or otherwise sensible, in practice the spell will generally be limited to 40,000 square feet of contiguous 5′ squares when needed to prevent abuse.
No requirement exists in the text, nor is one typically imposed in practice, for the caster to see the target floor space. In fact, the caster probably doesn’t even need line of effect to the floor space, since there’s no point of origin for the spell and the rules and “clarifications” establishing the need for a line of effect kind of assume one.
In short, the Cleric can ward the entire tunnel from outside. They should have to go to the tunnel, but don’t need to get any closer than necessary to touch it (and probably can touch the ground 80 feet away and just include that in the area as well). They do not need to be able to see the tunnel, and probably don’t need any doors in the tunnel to be open, either.