I carry AIWB, about 50% revolver and 50% S&W Third Generation pistol or Glock 26, 19/32. Maybe the occasional Beretta PX4 Storm Carry or Sig P365. AIWB had been my "go to" for 3 of the last 27 years.
For me, AIWB is the most comfortable, concealable, and functional carry methods. No printing. I can carry a larger gun. It's counterintuitive, but a larger gun can be more stable / comfortable. I have perfected my set up through trial and error. Even worked with holster makers to create the perfect holster when none existed. Started several threads about my custom Third Generation and revolver AIWB holsters. Waiting on two custom holsters for Smith & Wesson Models 10 and 19 snubs to arrive next week.
As for the sidecar question, no idea. Not a sidecar guy. I think it might be a fine way to carry a speed strip or two. Something flat. I like the idea. It's not for me. See if it works for you. Maybe a local holster maker can assist. Start a post if you find something that works. Would like to see it.
Chief38, I appreciate your post. AIWB is not for you. Fair enough. It was not my thing for 25+ years. Thought that there was no way anyone could carry AIWB comfortably. Then I LEARNED. I listened to people who carried AIWB for work and recreation. I explored the various purpose-built AIWB holsters and the reasons why they work, or do not work. Need to have the right length, maybe a claw or wedge, etc. I get the whole "not pointed at my junk" thing, to a degree. You can mitigate the risk with a proper holster. Revolvers and DAO or DA/SA pistols are another safeguard.
I carry comfortably all day. No problems sitting or driving. None. In fact, it is more comfortable than 3:00-4:00 for me and, again, no printing when seated, or if the wind blows against my shirt. I generally do not notice the gun. At least, not in a bad or uncomfortable way. All day at the desk, 9 hour drive, daily activities....all no problem.
Not sure why you chose a newb with no experience and a bad rig, a guy who is doing it wrong, to illustrate your position on AIWB. Sounds like a bad example of AIWB carry....but it is an example of guy who can learn from experience and eventually find something that works, or that nothing works. I am sure that AIWB does not work for some people. Maybe not for you. That's fair. You do not even have to try it. I am just here to offer a counterpoint. It can be safe, functional, and genuinely comfortable (for some people).
EDIT: Here's today's set up. A 4553TSW in custom Watchdog Tactical AIWB holster.