I actually think the plan is to sign Ostigard in the event we stay up.
the same with James and I’m still convinced Walsh will be signed
That may be the plan but whether it'll be feasible is highly doubtful.
Osti I reckon is likely to get quite a few offers both here and abroad (Germany most likely) and quite a few of those interested will be able to offer him more and probably be closer to being able to meet his high ambitions. Brighton may well want to keep him and offer him a new contract.
James is an option and although he'll be cheaper than his current contract he'll still want a fair wedge. Again likely to be some competition for his signature which probably won't work in our favour.
Walsh is becoming more feasible as time goes on with his injuries etc and those will also bring down what people will pay him. Others will still be keeping an eye on him but his injury might put them off a bit, plus his time with us, the familiarity and winning the league could help swing it in our favour.
I also can't see us signing both if we're assuming Sheaf is an obligation to buy, unless Kelly leaves I can't see a need to bring them both in and frankly I wouldn't want to do that. Plus with Eccles waiting in the wings and needing to step up soon or at least be given some game time to progress we'd potentially have Kelly, James Sheaf, Walsh and Eccles vying for those positions, not forgetting Hamer as well who i think could be moved higher up the pitch to play alongside and complement O'Hare but could still drop into the deeper position if needed.
When you consider we've got a left-sided hole to fill next season, could do with a striker or two upgrade to push on and probably a CB or two signing two CM's doesn't seem a priority.