I’ll go one by one here.
Not sure about the extra gear in your bags, it would depend on what gear it is and where it came from. As for ilvl, that is a decent way to see if it’s better or not, but at lower levels I would mainly just look at the stats. If a new piece of gear has more stats, just use that one instead. I think pressing shift and hovering over an item shows you the stat difference compared to the item you’re wearing in that slot. At max level ilvl is what you should mainly be going for, though specific items can still be better than a higher ilvl variant of another, especially with trinkets for example.
The zones scale from level X to level Y, meaning that anyone in that level range can level in any of those zones. For example, blasted lands used to be a level 60 zone, but now you could level there while being 34. The mobs are all the same level as you are, so you will not get one-shot. Same thing if you are level 60 and go to a lower level zone that scales up to 60, the mobs will be 60 there, so they always are the same level as you are. However, zones do not scale infinitely, there is a “scaling range” for each. For example, blasted land mobs do not scale above 60, and some higher level zones never scale below 60 for example. You can still see the level ranges of each zone when you open the map.
Professions work slightly differently since BfA. Basically, all the expansions have been divided into their own subcategories, and you can only get skill in those subcategories instead of leveling them from 1 to 800 for example. So, you could do 0 profession work in the earlier expansions and then start leveling your profession in BfA from 0 without any problems. Old recipes are in their own expansions category, and you need a certain amount of skill in that category to use them. So yes, you can skip leveling professions while leveling and focus on BfA professions once you’re max level, that’s fine.
There has been multiple, mainly because the max level has also gotten considerably higher, though the leveling process is something that Blizzard has been trying to move away from more and more by speeding it up and offering boosts and what have you.
If they’re from the same faction you share the tag on most monsters up to 5 players, meaning that if more than 5 players tag the same mob, the 6th player won’t get the rewards, but the others will. Doesn’t apply to some elites and bosses though. At the same time, if an opposing faction tags the mob before you do, you will not get credit, this only works when the other tagger is from your faction. Also, you will see whether or not you got credit for the kill by looking at the monsters level: is it red or grey? If it’s grey, you did not get the tag, and thus will not get the rewards either.
You can do that, sure. MoP was originally designed for characters from 85 to 90, nowadays I think you can do MoP content from 80 to 90 (refer to the scaling I talked about earlier). Cataclysm quests should also be from 80 to 90, so you can make a decision by yourself. Both are fine for leveling.
Any DPS spec will do. Tank specs are good for leveling but they work best at lower levels, the higher you get the weaker a tank is at leveling. I would say that just pick any DPS spec that you want, it doesn’t really matter and respeccing is very easy nowadays.
Sure, though I can’t tell you how much the mage playstyle has changed over the years. Mage is a good class at max level and definitely useful, so it’s not like it’s a bad choice. Try it out with your mage and see what you think about it.
Boralus/Dazar’alor are the new two cities that a lot of players are in, though on my server Stormwind is just as alive as Boralus is, IF is slightly more dead. Could just be your server, but I reckon that there’d still be players in the two new cities.
I don’t know, you’re going to have to try it out yourself, as this question is extremely subjective and I cannot tell you if they’ve done something that you wouldn’t like. Playing it yourself is the only way to find out. Personally, WoW has been in better places, but it’s still worth giving a shot.