Tonearm mount to the plinth vs arm board vs rotating arm board vs isolated tower

There is an interesting conundrum at play here.
I quoted Einstein quite deliberately.
”Does the Station stop at this Train?

Looking at this another way, if the Station and Train are rigidly coupled together AND moving. A passenger in the train looking at the station will observe no movement.  
The challenge for the TT designer is to keep the relative positions of the platter and arm board constant under dynamic conditions. Does it matter if they are moving about provided they are rigidly coupled.? Obviously yes if this movement is large or violent because the acceleration will impact the arm and cartridge. But what happens if the movement is small and benign? 
Keeping the two parts still relative to each other isn’t easy. There is a YouTube video posted by Peter of Soundsmith where he quotes an Ortofon engineer who states that movement of 0.005 micron can be traced by a cartridge.
This tiny amount is almost beyond comprehension.

By the way, smart guys (Atma-sphere, Richard, and Mark) can differ in opinion, which does not make the persons on the other side of the argument "amateurs".

Please don't ask me to explain the difference between 'Professional' and 'Amateur' in relation to turntables @lewm ......🧐

Keeping the two parts still relative to each other isn’t easy.

Perhaps not.......
But certainly not impossible 🤗

Amateur?    Hmm. 
I would point you to this post.....

 OMA (Oswalds Mill Audio) Debuts K3 Direct Drive Turntable—An AnalogPlanet Exclusive Michael Fremer  |  Feb 28, 2020

The OP in this thread, is asking about an idler.

@kanchi647
The idlers are one of the crudest, oldest form of turntable, and with them comes.....(based on my personal direct experiences)

The most vibrations, least isolation, the most noise.

That's what you are dealing with.

For these reasons I would recommend you try a design that works to eliminate those three (at least) design symptoms.

When done well they (idlers) can be quite good. I own a Jean Nantais Lenco that contains reference level (Parts) see my virtual system. His design was called the best turntable by blogger A. Salvatore at one time.

His armboard design specific to your thread question, has gone to great extent to reduce the symptoms mentioned above.

To give you and others reading an idea of how far he goes; here is a picture of mine showing the extent he goes to....to reduce vibrations, noise and provide for isolation. 

https://photos.app.goo.gl/MdYZd7sBGJ1cCHyC8

Similar cavities exist under the motor. Even with these huge cavities, it still weighs 100 lbs.