A few comments on your comments.
Firstly, the compressive strength of NEOs is not well understood. Many may be little more than self-gravitating rubble-piles, in which case your reaction engine will simply dig itself into a pit whilst ejecting boulders around it. Net result, multiple impactors on the same heading as the original body - which is Not Good.
The mathematics is simple to perform for a gravity deflector. Here the displacing force is dispersed over the whole body - no single point stresses to accommodate. What is not so simple to gauge is the mass of the inbound body. And knowing that quite well is crucial to the success of a gravity tractor. You are at least correct in estimating the need for multi-year duration burns to deflect plausible objects. That is neither impossible nor impractical.