1987 Service Pattern/Duplicate Letter Use

I recently took a look at the 1987 subway map and I wanted to talk about it in the hopes someone could offer insight.

Things of note were 5 Broadway services as was compelled by the North Side Manhattan Bridge closure. Track capacity is often talked about on these boards, how did service get along with the *B* *D* *N* *Q* *R*?

Along the same line was The Bridge at capacity with 4 services?

Why did BOTH and terminate at 34th/6av, why not send a service to terminate on the W4th 6ave express tracks or utilize 2nd ave? Send a few down to Grand st or Bustitute the station.

Why did the TA tolerate SO many duplicated letters? Considering at the time the MTA had P,T,U,V,W,X,Y letters available surely at least two of those were on enough rollsigns that it could've stood in for the B/D on Broadway, no?

Did the Bowery line warrant three services or was the TA running out of places to put trains?

Lastly was there something preventing the TA from installing an interlocking on the north landing of the Manhattan bridge so services could still reach both lines could still be reached?