How is a connector with only edge contacts as in MagSafe practically possible?
Power is irrelevant, the important parameter is current. At 85W and 16.5V the current is  just over 5A. One way the copy with it is having 2 connections for both \$GND\$ and \$V_+\$, so that each pin only has to carry 2.5A. Then the connections are low resistance. If both ends are properly shaped gold-plated connections like these can have a resistance of m\$\Omega\$s, let's say 10m\$\Omega\$. 2.5A \$\times\$ 10m\$\Omega\$ is a voltage drop of \$25mV\$, and a power loss of \$62mW\$. Nothing to worry about.