Hmmm, in my case ignition switch and lock cylinder is normal - at first i checked it in compliance with testing procedure above, and at second i disconnected switch from wire and make starting via paper clip - it have same result .
But i find one more possible cause of this situation - my friend said, that short circuit or coil breakdown at starter motor can be rise up current in circuit, starter looks like a operating, but hi current lead for "welding" main positive terminals at starter relay (on the fender in my case) and it can not be discharged after returning key from "start" position.
But i find once more strange things - all those troubles with stuck starter can be, if i use standard lock cylinder or shorting wire in dash. If i turn ignition on at key, and make start engine from jumper wire from positive AC terminal to control terminal of starter relay - starter never stay-on... Maybe, it is only "lucky situation", because i start engine in this way not often...
And why sometimes starter relay not charge-on when i start engine, when i turn key in start position? I heard only "click" from low-current relay in dash (this relay is unknown - wiring diagram is free for this relay). I think, that is "custom" feature from pre-owner.
Yesterday, my starter motor died - it stay-on once again, and it **** starter drive (probably). I can't open hood, drop down control wire and knocking to relay body in time... I failed my starter... Sh...t!!!
Today i will dismount starter for observing situation
))