A289: Yes, barring drawbacks casting is a mental-only action.
"If the wraith is concentrating on a spell or other effect and the host body is subject to an effect that would force a concentration check, the wraith makes the concentration check."
It is difficult to target the wraith during a passive possession. Creatures aren't usually aware of you (unless your mental only action creates a display or effect), so you can't be targeted for the most part. Effects that target possessing creatures can hit you and technically you would trigger an AOO if a creature knows you are there, but mostly they won't have a way to hit you.
A290: A staff you were wielding would still grant its bonus.
A291: The possessed creature does not count as being the same creature as the wraith, so abilities that only target self couldn't be used on the possessed creature. The exception is Path of the Ancestor, which can count a willing possessed creature as self. I haven't picked up Illusion yet and can't seem to locate the development draft, so can't comment on witchmarked.
A292: Yes, you manifest to take them into your possession. Being in possession should be sufficient to take it with you if putting it on takes longer than your manifested time (donning armor).
A293: No, the spirit blade can only possess its wielder, so object ride wouldn't apply.
A294: You burn a spell point each time and are limited to the actions you can take while passively possessing. Killing you would be annoying (barring a blacksmith with berserker sphere shattering the objects, leaving you expelled, dazed, and damaged. Force effects (Protection's barrier ward) will also stop you from going anywhere. If it was being abused, I would consider making multiple possessions in rapid succession strenuous, forcing escalating saves against fatigue maybe? I suppose the ranged possession haunt does not specify if the wraith travels to the target as part of the possession or accomplishes the effect at range (essentially teleporting inside the target); traveling to the target in some form would allow for the possibility of AOOs. Probably need to patch that one way or another.