To answer the first half of your questions:
The right-side 60-pin expansion slot is bespoke, and found on Mega Drive / Genesis 1 and 2 only. It is not present on the Genesis 3 or Nomad.
It exposes the address and data buses and a lot of other signals. In that respect it's very similar to the 86-pin expansion slot on the left side of an Amiga 500 (and the 56-pin trapdoor), but isn't compatible.
There is a second expansion slot, labelled EXT, found only on early Mega Drive 1s. It's a DE-9 female port similar to the front controller ports; is programmable and can act as a serial port. There are very few peripherals designed for this port, the main one being the Mega Modem, exclusive to Japan.
On the Sega Multi-Mega, the two 68000s are discrete as shown in this photograph from Wikipedia.
Your assumption about booting is wrong. If a Sega CD is installed, the memory map changes. The Sega CD takes ownership of $000000-$3FFFFF causing the system to boot into the Sega CD BIOS on power-on, and the 4 Megabyte cartridge slot address space becomes $400000-$7FFFFF.
According to Christian Schiller's Mega CD programming FAQ:
Both 68k processors run simultaneously. Only the Genesis 68k has access to the VDP and sound registers of the Genesis soundchip. That leaves only
three tasks for the SCD´s 68k:
- accessing the CD drive,
- accessing the custom chips and
The SCD´s 68k (or the special graphics chip) for example does all
the sprite/playfield zooming and rotation in games like Sonic CD or
Thunderhawk. Then the Genesis 68k simply downloads the pre-calulated
data via the connector port (which is, simply said, "just" a second
cartridge port) and displays it on screen.
Let´s imagine we load some bitmap
graphics from CD in order to display them onscreen. We´d need to do the following:
- give a load command to load the gfx file into the CD buffer
- copy the gfx from the cd buffer to the main RAM (512K?) of
the CD processor
- maybe rotate/zoom the gfx using the CD graphics chip...
- switch main RAM to Genesis CPU
- copy gfx data from main RAM to Genesis RAM
- copy gfx data from Genesis RAM to VDP RAM to display it