About 1: Not that much, but they get you going. Additionally you can download some free sound packs for the A4MK2 from https://www.elektron.se/soundpacks/ (at least a few of them are free) and when you search this forum here you will find other free resources.
About 2: Overbridge 2 public BETA supports the A4MK2.
Yes. Do note from the link provided above that you will also have to update your Analog Four’s firmware to a public beta version in order to use Overbridge. Think carefully about the risks involved in installing a beta version before doing so.
The applicable settings are in the MIDI CONFIG area of the Global settings menu. The channel numbers are irrelevant here because these MIDI messages are not sent on particular channels. Do read the manual carefully to understand the capabilities of the machine. You will need to do the following:
In the PORT CONFIG sub-menu, set INPUT FROM to “MIDI”.
In the SYNC sub-menu, activate CLOCK RECEIVE and TRANSPORT RECEIVE.
This is a bit more nuanced. Muting the tracks in the Digitone’s Mute Mode will stop sending MIDI messages to the AF Mk2, but that will not actually mute the AF’s sounds. Instead, you would have to send CC 7 (volume) messages from the DN’s MIDI sequencer tracks to the AF on the MIDI channel numbers that the AF’s tracks are set to in the GLOBAL > MIDI CONFIG > CHANNELS menu. I suspect it’s easier to manually mute the AF’s tracks using [FUNC] + the TRACK keys.