2) How many XP is there (roughly) in vanilla Oblivion?
I can answer this question, in part. Generally, your maximum level is dependent on your race, birth sign, and class choices, as each can modify your base skill and attribute values in vanilla Oblivion (you can actually read more details at http://www.uesp.net/wiki/Oblivion:Character_Creation). Basically, your leveling stops in vanilla Oblivion when all 8 of your attributes reach 100. Technically it can also stop when all of your major skills reach 100 as well (since you can't get the 10 skill increases required to "level up"), but there are ways around it to get a few more levels (using a custom
damage skill spell for instance. However, the hard cap is always when you max out all your attributes. You can still get the level up icon, but when you actually go to rest, you get a small message about how you have reached your peak growth, your level won't increase, and you won't get the screen to increase your attributes. So, in theory, the max level in vanilla Oblivion can range from 45 to 53.
That said, I have actually hit a max level on one of my characters (in vanilla, not using ObXP) using an efficient leveling strategy, which was actually 42 for a Bosmer with a Thief birth sign and Thief class. You can read more about efficient leveling at http://www.uesp.net/wiki/Oblivion:Efficient_Leveling, which also points out a lot of the flaws with vanilla Oblivion's default leveling system.
One more note. As I stated before in this thread, vanilla Oblivion does
not use an experience points system, so you do not get experience points for completing quests. The only rewards you get are either story progression, items, gold, or a combination of three. There are a few exceptions, like the Night Mother's Blessing for the Dark Brotherhood quest "Whodunit?," but those are few and far between (in fact, "Origin of the Grey Prince" is the only other one). So, you can raise your level, skills, and attributes to the max without ever completing a quest in the game (except for the tutorial quest, which you can't avoid in vanilla). Furthermore, you can can still continue to raise your skills to 100 (or beyond), even after hitting the max level, since the former is not strictly dependent on the latter.
As far as ObXP is concerned, I can also say that my current character is level is 39, and I haven't even touched half of the vanilla quests. However, most of my xp has been from dungeon crawling (i.e. kills) and lock picking, not from quests. I am also using OOO and MMM, which increases the number of locked containers and creature spawns significantly. More importantly, the level cap for ObXP (set in the INI file) is 50 by default, but again, you can hit that easily well before you finsh all the vanilla content. However, mine is set to 75, but I am using an uncapper mod that raises my attribute and skill caps from 100 to 200.
EDIT: In vanilla Oblivion, you can't ever realistically run out of spawns (unless you are trying to hit the same dungeon within 5 game days), as the in-game time it would take to hit all of the possible map locations would far exceed the respawn threshold, even using fast travel. So, it is technically impossible to blow through all content the vanilla game has to offer within the respawn threshold. Also, efficient leveling requires performing a lot of the "tedious" actions you mentioned. In my case, it was mindlessly swinging a sword at the unconscious prisoner from the vile lair plug-in to get a strength increase, as I don't generally do close quarters combat when I play (I prefer ranged sneak attacks). I can assure you that it is, in fact, very unappealing, and was one of leading reasons I started using ObXP exclusively.
EDiT2: I can also verify that you will hit the skill based level cap well before you finish all of the vanilla content, even if you avoid any non-quest activities (e.g. dungeon looting), provided you picked a class that has major skills you actually using during play (like Thief class for a stealth character, Warrior for a melee character, etc). In fact, the vanilla game levels you up too quickly in that regard imho.