1)
Purchasing possibe but sales only "regular" all JR groups(include JR-Tokai) shinkansen
also other line(not high speed Shinkansen tracks) train's reserved seat tickets and more.
Non-regular one means ex "Puratto Kodama"(special discount Tokaido shinkansen ticket name)
provided by JR Tokai Tours(JR Tokai's travel agency campany name) and more.(not so many)
http://www.japan-guide.com/search/?cx=015609760794546757367%3A7rvvvope...Most foreign travelers using "Japan Rail Pass(for nation wide valid for all JR areas)" usual
so for local Japanese tickets info still not so many translated in Eng(and other lang).
2-A)
JR(now 6 different railway campanies exist) means
not "one" National Railway(nation wide) company managed Japan as country official traffic division.
Natinal Railway period calling ‘“S(kokutetsu) as "JNR" until 1987 about 30 years ago.
https://en.wikipedia.org/wiki/Japan_Railways_Grouphttps://en.wikipedia.org/wiki/Japanese_National_RailwaysSo each JR groups(after 1987) based Natinal Railway legacies(tracks,station houses and more)
and basic rules(as low) of custumer but also sales/making original service for own areas now.
Regular shinkansen ticket is NOT special one so purchasing possible at another JR ticket office.
2-B)
"Tokaido Shinkansen" means JR Tokai's shinkansen[VŠ²ü](bullet train)
and for shinkansen tracks(for 285km/h max operating and closing section as no crossing) as line
from JR Tokyo stn(via Shinagawa) until Shin-Osaka(via Nagoya,Kyoto).
https://en.wikipedia.org/wiki/T%C5%8Dkaid%C5%8D_ShinkansenAfter Shin-Osaka will be line name change to "Sanyo Shinkansen" section managed by JR West.
"Tokaido (main) line" means NOT Shinkansen tracks using regular local commuter(around 130km/h max)
and freight trains mainly also a lot of crossing with as "conventional line(zairaisen[Ý—ˆü] in JP)".
Also line distance longer than Tokaido shinkansen tracks and managed 3 JR campanies
JR East,JR Tokai and JR West from Tokyo stn to Kobe stn(a little far Osaka).
https://en.wikipedia.org/wiki/T%C5%8Dkaid%C5%8D_Main_LineBoth lines are basically placed parallel like natinal road and Highway(as by]pass).
https://upload.wikimedia.org/wikipedia/commons/c/cb/TokaidoLineEastSta...Shinkansen mean New[V] main line[Š²ü] againt conventional line(zairaisen).
In Japan a lot of rail comanies (bigger like JR to small trams and more) and lines exist
also some own rules exist with so will be confused usual if even as born in Japan :)