I am following Approach 2: Single Slice exactly from this howto:
[url]http://people.freebsd.org/~rse/mirror/[/url]

Everything was done up to the first reboot, and it booted into the
degraded mirror perfectly. BUT, for some reason, the mirror automatically
added the first disk without me placing a new PC MBR onto the first disk
and switching GEOM mirror to auto-synchronization and adding the first disk.

Why?

Also, why are the consumers ad0 and ad2 instead of ad0s1 and ad2s1?

So after first reboot, it is doing the syncing on its own and I get this:
bash-2.05b# gmirror list
Geom name: gm0s1
State: DEGRADED
Components: 2
Balance: round-robin
Slice: 4096
Flags: NONE
GenID: 0
SyncID: 2
ID: 1399192993
Providers:
1. Name: mirror/gm0s1
Mediasize: 80026329088 (75G)
Sectorsize: 512
Mode: r5w4e1
Consumers:
1. Name: ad0
Mediasize: 80026361856 (75G)
Sectorsize: 512
Mode: r1w1e1
State: SYNCHRONIZING
Priority: 0
Flags: DIRTY, SYNCHRONIZING
GenID: 0
SyncID: 2
Synchronized: 6%
ID: 483524358
2. Name: ad2
Mediasize: 80026361856 (75G)
Sectorsize: 512
Mode: r1w1e1
State: ACTIVE
Priority: 0
Flags: DIRTY
GenID: 0
SyncID: 2
ID: 1568645277

Geom name: gm0s1.sync
Consumers:
1. Name: mirror/gm0s1
Mediasize: 80026329088 (75G)
Sectorsize: 512
Mode: r1w0e0