What happens if I've only 8 ball remaining, my opponent has his category object balls remaining. I called a pocket for 8 ball but didn't pocket it and it still remains on the table. Will my opponent get a ball in hand foul and can put cue ball anywhere on the table ?
No, your opponent can't put cue ball anywhere.
The rules says:
The shooter fails to strike one of their own object balls (or the 8 ball, if all of said object balls are already pocketed) with the cue ball, before other balls (if any) are contacted by the cue ball.
If these fouls are made, the ball can be placed anywhere on the table to prevent a player from making a purposeful foul to disadvantage the other player. So, the only way your opponent has is if you do not hit the 8 ball.
If it's a legal shot before the 8 ball, it's legal when you are on the 8 ball. In other words, the 8 ball doesn't change the rules for a foul. The only difference I've ever seen is the APA rule, that says a scratch (and only a scratch, no other kind of foul) when playing the 8 ball will lose the game.