If the tunneller is going to appear in the friendly half of the table it can be slated to arrive from turn two onwards. If it is slated to arrive in the enemy half of the table it can arrive anywhere from turn three onward.

When planning arrival, if a Tunneler formation will enter play on the opponent's half of the table, it must be Turn 2 or later. If it enters play within 45cm of the opponent's Blitz, it must be Turn 3 or later.

Tunneler

Tunnellers are set up touching their own side's table edge before the battle starts at the same time that spacecraft are setup (see EA 4.3.1). Any units transported in the tunneller should be placed to one side at this time too.

Secretly write down the location where the tunneller will surface at the same time and in the same manner that you record the coordinates of a drop zone (see EA 4.3.1). You must also secretly record when the tunneller will surface. If it is going to surface in your half of the table it may arrive from the second turn onwards. If it is going to surface in the opposing half of the table, it may arrive from turn three onwards.

Set up the tunneller at the start of the stated turn, before placing units with teleport, at the location you wrote down. Any units being transported are allowed to disembark immediately upon surfacing. Surfacing does not count as movement for the purposes of triggering overwatch fire. Disembarking triggers overwatch fire as normal.

If the tunneler surfaces on terrain that is impassable for it, under a friendly unit, or in an enemy zone of control then it is assumed that on-board sensor equipment will divert it towards another entry point. The unit should be moved by the opposing player to the nearest area where it can surface.

Formations of multiple tunnelers need only record one location where they will surface. Place the first unit on this location, all other units must be placed within 5cm of another unit that has already been placed and on the appropriate half of the table.

Tunnellers, and any units being transported in them, may take an action on the turn they appear.

Perhaps, but you can do the same with a Teleport formation and that's not pre-plotted. Has there been issues with something like Flayed Ones (scouts) abusing this ability? Or are you concerned more on a fluff/explanation level?

Given the 'nids strategy rating I'd much rather keep them close together

A 15cm radius is a huge area, I haven't even come close to using that much space when placing tunnelers. Most have been within a 15cm diameter.