Firstly I would like to start with the issue of the fluctuating BP. As you know and might have noted even on continuous monitors, there is a constant fluctuation of BP in every individual as a result of physiological mechanisms responding to the environmental factors as well as changes to the internal milieu (electrolytes, hormones, respiration etc).
From the renal RAS system, to the CNS cardiac centers, and the peripheral mechanisms like the carotid 'sensor' all are working in a delicately coordinated way in maintaining a control of the BP, NOT at a steady state but within a range of threshold.
So the difference between a hypertensive individual and a non hypertensive is NOT lack of fluctuating BP, but a failure of one or more of these mechanisms and hence failure of maintaining the equilibrium, allowing the BP to go up unchecked (Le Chattalier's principle of equilibrium)
Lisinopril an ACE inhibitor works at the RAS system, but as you rightly mentioned other factors might be at play, the adrenergic (emotional ) mechanisms (anxiety, stress, anger, joy, excitement, fright, fight,) all could still influence the balance.
Could it be based on this understanding, Doc that your wife's situation might be vice versa (meaning could it be that it is 'subconscious' anxiety/ agitation etc that is influencing the fluctuation rather than the fluctuation causing the agitation)
Logically, increased blood pressure is a RESPONSE of the physiological changes in the homeostasis brought about by both internal and environmental factors, and might in itself influence other physiological endpoints like heart rate, renal function etc (over long term also anatomical changes like atherosclerosis). Doc, we know this as a basic understanding of physiology.
It is difficult to argue it would initiate any changes in the psyche, except if it does that by affecting the anatomy, like in ischaemia leading to anatomical changes in neuronal pathways, it produces other symptoms, that are easily recognized like memory impairment etc.( MRI might be indicated to investigate this)
Lisinopril has a long half life (12 hrs accumulated), it will be unwise to give it 8 hourly. If 2.5 mg is not keeping the BP within a 'desirable range', then it could be increased to 5 mg or changed to Angiotensin Receptor blocker (ARBs) like Irbesartan or more a Direct Renin activity inhibitor like Aliskeren, ( They have more benefits in PCOS, and both classes are more renoprotective as well as cardioprotective than ACE inhibitors, in addition to antihypertensive properties)