Advanced problem: how do you avoid accidentally killing someone while targeting this?
You're just talking about shooting "at" someone's head, you're talking about positioning their head in 3D space and shooting so precisely (through potentially layers of brick, metal brick ties, lath etc.) that you hit only inside their head and don't leave any marks or burns on the skin.
We don't have the technology to reliably do this today at range.
Use radar for targeting. WiFi has been used to see peoples bodies through walls, 10GHz has been used in radars, the same beam that destroys can be used to image the target. You may not even need to do imaging, just look for resonance or increased energy absorption to indicate the beam is on target.
Beamforming is a solved problem, using the interference of multiple antennas' energy to resonate at exactly the right location for maximum strength. Why do you think this technology isn't reliable?
Because you are trying to hit a 15cm by 15cm sphere in 3D space that you cannot see.
That it could maybe be done under ideal conditions doesn't explain how you do it when the target is mobile (but let's be generous and say you assume they're in bed), and you need to accurately visualize where they are through an unspecified amount of intermediate interfering elements (people's bedrooms aren't glass boxes).
Foil wall insulation would disrupt an attempt at microwave beamforming - so how is targeting being achieved so precisely as to be undetectable and cause no other environmental effects?
That's not the point: it's the "undetectable" part that's the problem.
You could definitely point a bunch of microwave equipment at someone's room and cause some damage, that's not in question - but to somehow cause a bunch of neurological problems, but not leave thermal burns, or heat up jewellery, or fry electronics?
And this isn't a "with todays tech" issue either. This is all being proposed as being totally possible with 90s-era technology. The sheer amount of wireless tech innovation and availability since then has been enormous - what feels possible today is ignoring that in the 90s a cellphone looked like a brick with an antenna on it. Solid-state microwave elements weren't COTS parts you bought from China in bulk.
We're talking about doing a lot of very difficult to do things, with equipment and power outputs which would require substantial draw from either residential electricity supply systems, or banks of lead-acid batteries (no lithium ion in that age either - so you have a transport and weight problem).
The more disperse you propose the signal is, the less plausible it gets too - at some point your number of field teams and the size of the equipment is going up and up and up and all of this is happening within line-of-sight (or near enough) of a target - microwaves don't penetrate multiple buildings.
Advanced problem: how do you avoid accidentally killing someone while targeting this?
You're just talking about shooting "at" someone's head, you're talking about positioning their head in 3D space and shooting so precisely (through potentially layers of brick, metal brick ties, lath etc.) that you hit only inside their head and don't leave any marks or burns on the skin.
We don't have the technology to reliably do this today at range.