Both USA and USSR tested and operated nuclear and non nuclear anti satellite weapons in '60es and '70es. Conventional systems, a F-15 launched ASAT missile for USA and a (potentially reusable) killer satellite named IS (istrebnitel sputnik - satellite killer) for USSR were declared operational in '70es. And a nuclear blast in low orbit would EMP both everything in orbit in LOS and everything below it. (yes, you need atmosphere for EMP, there is quite enough atmosphere even at 1000km altitude of Starfish Prime)
USA had almost lost its ASAT capabilities by discontinuing systems. It is unknown whether Russians still keep launch prepared boosters with IS as payload, since they scavenged a lot of military surplus booster for commercial use in '90es.
Last year test was forcing a ABM missile to ASAT duty, nearly outside its engagement envelope, and it still worked. Even the freaking Chinese did a sucessfull ASAT test a year or so before that.
Let me put this all in one simple sentence. It was technically possibly to shoot down satellites in orbit since '60es, and systems were tested in practice. Storing warheads in orbit would have been worst possible choice. FOBS was theoretically better until space based surveillance made it obsolete.
Not signing OST would be very destabilizing. In potential crisis both sides would view all existing satellites and confirmed non ICBM launches as suspicious and dangerous, and could likely cause a preventive attack on enemy satellites causing war they wish to avoid. It would lower the threshold of starting nuclear war much lower.
Though its already very low when we are talking about satellites. USA and USSR operated launch warning satellites in high orbits and in GEO. They were considered so crucial to early warning that any attack on them would have likely caused immediate nuclear escalation. Yes, hitting a target in GEO is far difficult than one in LEO, though their very nature forces them to stay in same orbit, warning satellite that isnt looking at what is supposed to isnt worth that much. And when you can launch GEO platforms of your own, you have all technical know how to build a ASAT capable of hurting others GEO assets.
Ahem. You cant change orbits constantly, as it consumes limited supply of maneuvering fuel satellite has on board, once your out you cant maintain orbit or change it in need, and you will run out very quickly if you want to be unpredictable. Stuff you said about mass drivers and asteroids is still pure science fiction, outside our current technical capabilities, and not even worth a laugh in '60es or '70es. It cant be done with current technology and space launch capabilities, and not even keeping Saturn V could help a bit. You might as well say "omg what could we do if Commies started getting starships and weapons from Cardassians", it is as much plausible.