OK, but I can still give you another option,
how about this Ferry fr Tomakomai(苫小牧) to Hachinohe(八戸)?
http://www.silverferry.jp/route_guide/http://www.silverferry.jp/schedule/You will be able to catch Shinkansen "Hayabusa 4"
dep 6:41 fr JR Hachinohe stn, arr Sendai 7:50(2min stop).
http://www.eki-net.com/pc/jreast-shinkansen-reservation/english/wb/com...Yr suggested plan "Hayabusa 10" will be stopping Sedai 9:28(2min stop).
BUT, Hachinohe Ferry port is not near by Shinkansen stn, located in mid town of Hachinohe City.
https://goo.gl/maps/23i7JU4ExsP2So if taken this ferrry arr is too early morning, so no public bus and local train,
you have to catch a taxi about 20min (about 3000Y), or hitchhike or share a taxi fare,
try to asking/catching someone while ferry?
Or, waiting at Ferry terminal building until local train(JR Hachinohe line) arr time,
nearest stn HonHachinohe stn dep 6:58,
catch a taxi about 10min fr Ferry terminal. (about 1800Y)
https://goo.gl/maps/BcHgsMzbYNsLocal train will be stopping Shinkansen stn Hachinohe 7:07,
then catch "Hayabusa 8" dep 7:17 arr Sendai 8:55(1 min stop).
http://www.jreast.co.jp/e/stations/e1230.htmlhttp://www.eki-net.com/pc/jreast-shinkansen-reservation/english/wb/com...BTW, when?
If after mid next MARCH 2018, NEW timetables of "JR" updated yet.