Marin MTB - a question aboutt the steering

thats a marin bear valley by the way : )

Yes it does have one.
I'm afraid I don't get how it might interfere though...

no neither do i but i would minimise the number of moving parts in the general area surrounding your problem. also the fact that it does have an extender might have well been the cause of your issue if you had a heavy person leaning on them and putting more strain on the head tube than it was intended to have.

A few weeks ago I wrote " you never know there could be other problems ahead
that will need more experienced help "

I can't resist saying " I told you so "

This to someone who was not prepared to pay London bike shop prices to remove
some pedals . Fair enough , but it won't cost anything to go back to the shop , get
an opinion and perhaps an estimate for fitting a new headset .
It could have been on there for 20 years and is probably dried out

I don't see that the extender would produce a significant increase in load on
the bearings , if anything the more upright riding position throws more of the
riders weight on the saddle and reduces loads on wrists/hands