The numbers will never be perfect. The State of West Virginia has done well in correcting an historic misconception. After over a century, numbers of volunteers from other states were removed from the lists of WV Unionists. Also numbers who were on the militia rolls of what became WV who served in CSA units were removed. After complete editing, the numbers of Union and CSA became nearly a "push". That had corrected a very long standing understanding that Unionists outnumbered Rebs by over 2 to 1.We do have scholarly estimate for troops from the above states other then Maryland. Per Freeling Kentucky was 25k Confederate vs 50k Union, Missouri Historical. 30k Union vs 130 K Union. Wv approximately 22k vs 22k. So different results from different border states even Tennessee per Current was 42k Union and Ark 10k Union vs more on the Confederate Army.
Maryland apparently is somewhat of a mystery.
There are issues with other state numbers. The numbers will never tell a complete story. Missouri, and to lesser extent some other states, had its own war. No accounting just two sides there. Tennessee is an issue. A lot of blue clad Tennesseans were not in front line units, while almost all the Rebs from Tennessee were. About the same for Kentucky.