No, it isn’t totally different everyday. The LOTH revolves around a four week Psaltar, so the prayers and psalms are basically different everyday for four weeks and then the cycle starts over. The exception to this is there are also special prayers for the various feast days and holy days.
A long time.
Here is a site with a short history.
Yes, it was originally prayed by religious. A lot of that has to do with practical reasons, time and literacy rates. As for how hard it is to learn, it does take a bit of getting used to the flow and pattern, but it really isn’t that difficult. It helps if you have a good guide or a real person to help answer questions. And the thing to remember while you are learning, is that it isn’t like God won’t hear your prayers if they aren’t exactly right, He is perfectly happy with or feeble and confuse attempts.
The Liturgy of the Hours, Divine Office, and Breviary are all essentially the same. Sometime you will hear people use one term more for the older version (kind of like the EF vs. OF), but they are pretty much interchangeable.
The LOTH is the official prayer of the Church, second only to the Mass. It is what religious pray, what priests and deacons pray, and what many laity pray.
When you hear people say the rosary is “only a devotional” I think they are often reacting to the attitude that ALL Catholics MUST pray the rosary or they aren’t “good” Catholics. If anything, it really should be the other way around since the LOTH is the “official” prayer of the Church.
In real life though, we don’t have time to participate in every devotion out there, so we use what draws us most to God. For many, that is the rosary, for others that is the LOTH, or even something else. I wouldn’t get caught up in which is “better.”