Hi Qowpel, & welcome.  No worries about dumb questions here.  We're all living and learning all the time.  
 
Regarding the persistence of IVM in tissues, I was surprised to learn this just the other day. 
 
https://www.annualre...5.100191.002305
 
Apparently, for onchocerciasis a single dose once a year is all it takes.  
 
"The investigators concluded that a dosage of 150 u/kg, given yearly, represented the best regimen in terms of efficacy and safety."
 
Don't know what the long term implications are, but IVM has been around for several decades, with over a billion doses given for parasitic disease in Africa.  I'd think if there were issues with longevity, someone would have noticed.  
 
MERCK's monograph on Stromectol (their brand name for ivermectin)... 
 
https://www.merck.ca...MECTOL-PM_E.pdf
 
...doesn't have any ominous indicators I can find.  Keep in mind, many of the side effects mentioned occur due to the parasitic die-off, and in these cases the drug itself is not what is causing the symptoms.  
 
Personally, I feel the risk/reward ratio favors going with the IVM prophylaxis.  I'm 64 and in good health.  At my age, I no longer fear death like I once did, but chronic illness is now on the front burner for me.  I really don't want to have to deal with pulmonary fibrosis or cardiomyopathy the last 10-20 years of my life.  The brain fog is also a major concern, if this turns out to be chronic in some patients.  
 
Can't advise others on this decision, but it's a no-brainer for me.  I've been on IVM prophylaxis since November, without issues.  Getting out from under the coronavirus black cloud has changed my entire outlook on life.  
 
Best of Luck, & hope to see you posting again soon.