When in the webway, the formation is off-board. There is no distance to measure. They wouldn't count as being within within 5 or 10cm for ZoC. If they were, then the opposite would be true as well, and units in the webway would project ZoC out of the gate, forcing the opponent away, and since there is no way to assault into a portal, that ZoC would be impenetrable. Also, if you were to count the units in the webway as being in the same location as the gate, then you'd run into issues with things like 30cm distance for Rallying.
Coming on the board through a gate covered by someone's ZoC would require entering that ZoC.
After that, you follow the normal rules.
AgeingHippy wrote:
If a Wraithgate is covered by the ZoC of some scouts then I assume any formation coming out of the gate must either assault them or move directly out of the scout's ZoC?
The exiting formation would have to assault. That would apply to any ZoC, not just a scout's extended ZoC. Normal units can block a gate just fine.
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What if the Wraithgate is surrounded by scouts or perhaps up against the edge of the table with the table side surrounded by scouts (and still in ZoC of scouts)? I assume in this case the emerging Eldar formation *must* assault the scouts and cannot simply bypass them (in the case where they may have enough movement to end outside the scouts ZoC)?
Yep.
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Next, assuming the Eldar forces fail their activation, and decide to take a Hold action, I assume they then cannot enter play as they would then move and in fact end their move in the scout's ZoC which I imagine to be illegal?
Correct. Without an Engage action, the off-board formation would not be able to enter an enemy ZoC. A move on a Hold action could not come on the board at all. Since the formation can't shoot from the Webway, that pretty much leaves them to Regroup off-board as the only real option for their Hold action.
stompzilla wrote:
It's also generally accepted that if there's not enough room on the table for the models to be physically placed then the Eldar (Or anyone trying to use a portal - Necrons etc) can't use the portal.
I would say they could use the portal, but any units that couldn't fit on board would be out of formation and destroyed per 1.7.4.. Obviously, that's a substantial disadvantage in trying to assault out of the portal but there might be a situation desperate enough to make it worth sacrificing that portion of the formation.