Heart Failure with Preserved Ejection Fraction (HFpEF): The patient's symptoms of progressive shortness of breath (SOB) and fatigue, along with an elevated BNP of 1000 ng/L, are highly suggestive of heart failure. The absence of peripheral edema and crackles, along with a non-distended JVP and clear CXR, does not rule out HFpEF, as these patients may not always present with typical signs of fluid overload.

Atrial Fibrillation or Other Tachyarrhythmias: The presence of frequent PVCs on ECG could be a precursor to or associated with atrial fibrillation or other tachyarrhythmias, which can cause SOB and fatigue. The elevated BNP also supports this possibility.
Anemia or Other Chronic Conditions: Anemia or other chronic conditions (e.g., chronic kidney disease, thyroid disorders) could explain the patient's fatigue and SOB, although the elevated BNP points more towards a cardiac etiology.

Pulmonary Embolism (PE): Although the D-dimer is only slightly positive and other signs like peripheral edema are absent, PE is a diagnosis that cannot be missed due to its high mortality rate. The lack of typical signs does not rule out PE, especially in elderly patients who may present atypically.
Cardiac Tamponade: This condition can present with fatigue and SOB, and although the JVP is non-distended, cardiac tamponade can occur without significant elevation of JVP if the patient is hypovolemic or if the tamponade develops rapidly.

Constrictive Pericarditis: This condition can mimic heart failure and present with similar symptoms, including SOB and fatigue. However, it is less common and typically requires specific imaging findings for diagnosis.
Restrictive Cardiomyopathy: Similar to constrictive pericarditis, restrictive cardiomyopathy can present with heart failure symptoms without significant fluid overload signs. It is a rare condition that might be considered if other diagnoses are ruled out.