Can one whose heart Allah has sealed be saved and guided, and what is the path to that, and is there any sin that Allah does not forgive?

No one knows their ultimate end, and the servant is obligated to obey Allah, fulfill His commands, and avoid His prohibitions. Despairing of guidance under the pretext of a sealed heart is a deception from Satan. The only sin Allah does not forgive except through repentance is associating partners with Him. Not seeing a swift answer does not mean a lack of guidance, and there may be wisdoms or impediments behind a delayed answer. Whoever finds hardness in their heart should supplicate, seek forgiveness, and strive against their self for guidance.