> how important was R&D done in the name of human space exploration to the development of ICBM capabilities?
Not very important. By the time of the Gemini program, ICBMs had reached the capabilities needed. All the human rated launch vehicles after that were developed for manned spaceflight exclusively (unlike Mercury and Gemini programs which used missiles) and didn't have a lot of overlap with ICBM development.
There might be other areas of military interest that overlap manned space flight efforts, though.
Not very important. By the time of the Gemini program, ICBMs had reached the capabilities needed. All the human rated launch vehicles after that were developed for manned spaceflight exclusively (unlike Mercury and Gemini programs which used missiles) and didn't have a lot of overlap with ICBM development.
There might be other areas of military interest that overlap manned space flight efforts, though.