I want to have the eggs in lockdown before they start to hatch. For me, that means external pip. The risk with having the incubator open after external pip is that the membrane might dry out and shrink wrap the chick. That does not happen very often but it can. To me it is best to avoid that risk when I can.
Eggs can hatch two full days or more early or late and the chicks still be viable. That's why we lockdown three days before the 21 day window, just in case some are that early. If they have not external pipped I don't worry about it, they are not likely to shrink wrap. As long as you don't see an external pip I would not worry about candling them.
If I have an emergency during hatch and need to open the incubator and let the moisture out, I do and take care of the problem. I accept that the risk is there of shrink wrapping some of the eggs if they have pipped but the risk of that happening is usually fairly low. I don't casually open the incubator during lockdown just because it is fun though. I recognize there is a risk but if the emergency warrants it I'll take that risk.
In your situation if I did not see an external pip I'd put down the shelf liner and candle the eggs, discarding the ones that have not developed. If I saw an external pip I would not. Have a spray bottle of warm water handy to mist any eggs that surprise you with an external pip.
Good luck!