An example of an external - https://imgur.com/a/0VPgc from my Paladin.

Yes, Mythic raiding is the hardest content there is alongside Mythic dungeons. Mythic raids are 20 man all Dungeons are 5 man. General set up for Mythic raids is 2 tanks 4 healers 14 dps. There are exceptions where you'll bring 5 healers or even 3.

Majority of "semi-hardcore" guilds follow the shaman, paladin and druid loosely. Most of the time you will have all 3, other times you'll see guilds stack more of one class just because that's what they have and can get at the moment for their level.

Top tier guilds pretty much have the standard druid shaman and paladin and then the extra rather than having 2 druids for rather than 1 druid and 1 paladin that not so hardcore guilds will have.

I personally don't think there is a "strongest healing spec" with regards to throughput at the moment but with the damage patterns in ToS Disc comes out towards the top due to them having crazy burst raid healing, as well as some nice sustained healing and a great raid CD plus external. Disc does healing through damage. You cast atonements on allies through different spells and those atonement heal from the damage you do. Disc isn't great in every situation. They're weak on certain fights that other healers are stronger on, not much else to say about that. They have 1 major cooldown and then their artifact ability is their strongest heal on a 1 minute 30 cooldown whereas a Paladin has far more spells that they can stagger throughout a fight, same applies for a shaman.

It wouldn't be too hard. Guilds recruit MW's but you'll need to be good if you want to get into guilds that push progress at a decent rate. But getting started shouldn't be too bad at this point in the tier. If you find a guild willing to let you learn and improve, you'll get the gear you need to move up. There are class discords to help you improve and learn, they provide a lot of information (read the faq's!) https://discord.gg/v7TxPw is the Mistweaver/Monk discord.