No chance of getting Niasse back, he might be on a free but he'll be able to go to Turkey and get £15-20k a week comfortably. Can't see us being able to offer Sonny enough either if Luton can't. Plus he's a left-footed centre half and we've got Greaves so it wouldn't make sense either.
Really want to see us get Slater back, difficult to find a player with his attributes in the deeper midfielder role. Not fussed about Whyte though, he's levels below KLP & Wilks, and they're only getting better. He also failed to score but got 1 assist for Cardiff in 32 games for them (roughly 1400 minutes) Screams of someone who's not good enough for the level. Plus, they signed him for £2m and will want to recoup a big chunk of that. I liked him in L1, and I'd wouldn't mind if the fee was £300/400k but can't see Cardiff doing that.
Striker wise, Dion Charles is the one. He fits our system perfectly but still has plenty to work on and improvement in him. He'll be about £1m but he's got 12 months left, and there's no chance he'll renew so Accrington will want to flog him as it's huge money to them.
Players like Niasse will go where they are wanted, it'll depend on where the offers come from.
I'd take Slater, but I think you're underplaying Whyte a little. He's a winger, he was inconsistent and he had to go out on loan to rediscover his form. Like you say, he won't get in ahead of KLP/Wilks from the off, but like Mayer he'd be decent cover on a free transfer. Cardiff don't want to keep him and it would free up some wages, I don't think they'd be inflexible with the fee. He's clearly too good for League One and players like that make decent cover at Championship level.
I agree about Charles, but the more I look at his situation, he's got the pick of a lot of Championship clubs who can offer him more money. Can we be fair to Josh Magennis AND guarantee Charles a starting berth to sway his decision? Probably not and the fee would be at least £1m. I think Pigott at Wimbledon would be another option, or Paul Mullin at Cambridge United who's had an unbelievable season (32 league goals, 34 in all comps). The latter has played at a division lower, but he'd be much cheaper and as far as I know he's out of contract.