1: I play other games, most content in this game is based on being throw away content so if your not raiding there is not that much to do. Before a new raidtire I do however make sure I have enough materia for incoming gear.
2: I would say every 6 month when the new raidtire comes.
3: They do try new mechanics every now and again, ones that are completely new and innovative are sadly few and far between especially outside raids and trials.
4: It changes from often and many hours per day during patches where the content interests me to days without even playing during patches with contain content that does not interest me. Having a raidgroup or not heavily influences this, an example is that during 4.1 I did not have a raidgroup and due to that hardly ever logged in, during most of 3.x and now again in 4.2 I will have a group (built by most of those I raided with during 3.x) and will therefor login more often to make sure I can play my role well enough and also attempt to improve my gameplay in order to benefit the group.
5: Their plan is every 3rd month, even has usually been for raiders with odd more for casuals, this has however changed with them seemingly focusing on raiders in even and hardcore in odd... kinda screwing over the casual players, if this decision ends up biting them in the ass we will have to wait and see.

The endgame is savage raids and ultimate fights, the dungeons they add are to easy while they also add to few of them now, content like Eureka (relic weapon related) as well as the new version of potd seems to be pushed back further then they should be. Eureka for one should have been in 4.1 but was pushed to 4.25 I think, this is a poor decision imho, the new version of potd is rumored to be planed to come in the later part of 4.x which is also a bad idea since if it is done right would provide content with a high replay value.
Housing is something that people either ignore or love, I love it and have my own medium size house, something that would be nice is if they added more of a reason to have a house beside playing the Sims FFXIV edition. But if they wish to add more functions to the houses then they also have to make them available for everyone that want one.