Speaking as a musician myself Willem.
i feel that in answer to your question, the reason if any of these old 60s garage recordings do sound demented.... is this...
(In regards to The Keggs, I know what you mean by demented, maybe the wrong word to use, however I know what you're trying to say.)
Ok.
Back in the day, many of these kids, had never been in a recording studio and probably were nervous as hell to record, they probably only had a whole hour to record two songs, guitars can go outta tune easy, getting a guitar in tune back then was a skill conferring they didn't have electric tuners like today.... when I try tuning a guitar by ear, it's always wrong.
So being in the studio for hour probably meant they had one practise of the songs in the studio before taping the 45, often tracking the thing live and over dubbing the lead vocals or lead guitar afterwards.
I feel the vibe we hear of these great garage 45s... is a mix of nervousness, determination to do the very best given the time and circumstances and obviously being teenage boys there was a great deal of natural frustration in the mix too.
I think it is wonderful..... how exciting for these teenagers to go into a studio for the first time to cut a vinyl record, what a BUZZ it must have been for them.
The above is exactly how I record and feel when I have created my own 45s.
it's such a thrill.