This is sort of a tricky question... It really depends on how much time you put in. I finished the main story quests for ARR in a little under 2 months, but I was sharing my ps4 with my gf and I was doing all the side quests and getting distracted with other things as well. So if you wish to just push through the main story quests to get to the end of ARR content, then I'd say if you play about 5 hours a day and just focus on the necessities, you should be through in a few weeks. 2-3 maybe. But with that, you'll be passing up SOOO much content in the side story quests, etc. For me, I dislike leaving quests to do later. I prefer to clear out an area with the quests there before moving on (hence why it took me a while to clear ARR content, and why I'm still doing HW content).

Yes, the MSQ in ARR aren't too difficult. There are a few dungeons/trials that are a bit challenging, but as a tank, you shouldn't have problems finding quick queue times in DF. I would suggest you read some guides or watch some video's on some of the more challenging dungeons/trials. DF is hit or miss with the people you get. Some are very nice, patient and helpful. Some are... let's just say not as accommodating. Don't let those people get to you. I will say though, as a tank, you'll have to deal with some impatient people regardless. I feel I'm pretty experienced/good at tanking and I still get people complaining about stuff every once in a while. Best advice I can give for you is, if you enter a new dungeon or trial, say it. Then no one can blame you for not knowing what the mechanics are if they are too impatient to wait before pulling (but once again, as you are tank, you are the one to start things off). By the time you reach 8 man dungeons, you should have a very good gist of how to tank correctly.

Last, I think yes. HW is totally worth the effort. So much more content and things to do. I'm having a blast in it. I've found many of the dungeons in HW require you to follow stricter mechanics to get through the fights, but I haven't had much problem with it. Once again, be sure to ask when you enter a new dungeon or trial if there's anything you should know.