TL;DR: Don't bother booking in advance, just show up and take an unreserved train.
The fastest & easiest way from Narita to Sendai is to fly. However, schedules have been cut due to COVID and ANA currently only flies 2x/week (!), which is unlikely to work for you.
Historically, no discounts were given for booking in advance on JR, the only thing this got you was a guaranteed seat. However, outside peak periods like holidays, there are always unreserved seats available. You could make a reservation and change it for free if it looks like you're not going to make it, but this change has to be done in person at a station and before the original train departs, so you'd be out of luck if the flight is delayed.
These days, some discounted fares up to 30% are available, but the discounted fares typically have zero flexibility, so they are not a great fit for your situation.
So my recommendation is, just show up at the station and take the next unreserved train, they run every 20 minutes or so from Tokyo to Sendai.
Pro tip: instead of the obvious choice of JR Narita Express to Tokyo Station, take the Keisei Skyliner to Ueno and board the Shinkansen there. This is cheaper, faster, and Ueno is a smaller and more navigable station too compared to the 3D labyrinth that is Tokyo Station.