1. why did Isshin not protect his family in the beginning from those hollows (ex: Inoue Sora)?
i think it's because Isshin already knew Ichigo had the powers to take it on. because if you think about it, if Isshin is an acting shinigami, he should be able to sense the hollows, no? and since his family has such high spiritual power, he should be expecting a certain amount of hollows to show up in his area, on a more or less regular basis. if those hollows started decreasing and you add that with the fact that shinigamis can detect each other's life force/ki emitting from them (cant remember the exact term); then Isshin SHOULD already know by then that Ichigo's already at the level to take out low level hollows. If anything were to happen, he would step in.
2. Where did Ichigo receive his powers from? Kaien? Masaki? Isshin? Urahara?
At first, he received TEMPORARY powers from Rukia. those powers awakened his true power after Byakuya stripped him of Rukia's. although we cannot say for sure wether or not he is the reincarnation of Kaien or not, we do know 2 distinctive facts. 1, Ichigo does have a hollow presence within him. 2, he does indeed look like Kaien. (that theory we shouldn't touch untill there is more to back it up).
Masaki, the mother. when she went to save Ichigo, it was never revealed what she tried to save Ichigo from; the slipperyness of the grass which may have cause little Ichigo to slip, fall and drown in the river, or from the lure of Grand Fisher. Therefore we cannot conclude wether or not she has any spiritual powers or not (remember, Yuzu has no spiritual power either).
Isshin, the father. it is quite obvious now that he does possess Shinigami power, and on top of that, quite a sum of it. as someone stated earlier, Urahara did mention the fact that Ichigo has his "own" power. personally i dont believe in the genetics theory. reasons: Yuzu and Karin. they are twins (apparently) but one possesses spiritual power and one does not. both came from the same parents at the same time. if the powers are genetic, why do they both not have that same amount of power? another flaw in this theory's theory. twins are suppose to have a certain connection with each other, so how come Yuzu cannot sense, even the tinyest bit, when hollows are near by?
Urahara, bucket hat. Ichigo definatly awakened his powers from training with Urahara, but most definately not to the point where it would influence the amount that he has. like the case with Ichigo and his friends, sure he can influence them to awaken their spiritual power, but not the ammount because they too had to reawaken their powers with training provided by Yoruichi.
3. Isshin, a shinigami o_O?
Untill now, Kubo-sensei has decided to keep this fact a secret (although he's dropped clues here and there). Now the question occurs, how strong is Isshin, why does he have a vice captain's band on his arm, and why does he carry a captain's jacket?
a. How strong is Isshin?
even though we cannot prove that he is a captain, i think we have a good idea that he might be as strong as a vice. why? see below
b. Why does he have a vice captain's band on his arm?
i agree with whomever said so earlier, i believe that Isshin was Urahara's vice captain. It explains why he's in Karakura and has knowledge of where Ichigo goes and what he does. It also explains why Mayuri's vice is his daughter (if Mayuri took over, why would his daughter take over vice suddenly?).
c. Why does he carry a captain's jacket?
in my opinion, that captain's jacket may very well belong to Urahara Kisuke. considering Urahara lives in Karakura and is within walking distance from the Kurosaki's. it also backs up the theory that Isshin is Urahara's vice-captain.
4. ____ must be the reincarnation of _____!
we cannot say nothing for sure. for all we know Ichigo maybe Kaien's body host (ichigo may have died during the 1st GF encounter and is now going through a whole angel sanctuary phase). also for girls super_lucky's crack theory on them, i must disagree, if Tatsuki was the reincarnation of a shinigami, she would've had the power to fight off that school raid by the hollows expecialy if Ichigo has enough spiritual power to awaken Orihime & Chad. i mean, Tatsuki has been with him since he was how old? before his mother died, meaning more than 6yrs. how can Orihime & Chad who have been with him for probably less than a year gain more spiritual power than Tatsuki (whom is said to be a reincarnation) who has been with Ichigo for about half her life?
well thats my.... well lets say 20$ on the crack theories. its 1am and i think i missed a lot, probably not making that much sense either, but there it is.
btw: Gaia Commission Arts are uploaded on my website. that and theres a commission section that allows you to order commissioned cosplay outfits from me :3