This now takes place at Longfield Junction, near Gravesend, much nearer to London, following the opening of the first half of the express railway from the Channel Tunnel to London. Also accompanied by a marked increase in speed.

Regarding Channel Tunnel mid-tunnel stops, I've never known one to happen.

Interestingly no one ever has a concern about the sections of the London Underground that pass under the Thames, usually so close to the riverbed that repair works can be done from a barge in the river working downwards, and maintenance crews walking through at night can hear ships propellers overhead. If you are on the Underground station platforms just before a line goes under the river you can see the big watertight gates that were installed in WW2 in case these tunnels were breached during an air raid. They were never needed but are still occasionally tested.