On which point(s) in a jet engine does the reaction force act?
The thrust produced by the engine is due to the net resultant of various forces acting on various surfaces of the engine. The thrust produced is a function of mass flow rate and the change in velocity ($T = \dot{m} (V_{e}-V_{\inf})$). So, both of these have to be taken into account, rather than acceleration alone. The location of maximum thrust generated varies with the type of engine.


For high bypass jet engines like those used by the modern commercial airliners, most of the thrust is produced by the bypass fan. Though the acceleration is not the greatest here, mass x acceleration is- so this is where the thrust is maximum. The maximum reaction forces are applied there
In case of pure turbojets, the (almost all of the) thrust is produced by the core. For low bypass turbofans, its somewhere in the middle, with the lion's share of thrust produced by the core.


Note that most of the reactive force is applied on the diffuser and the compressor due to the high pressure and the forward facing area (due to cross sectional variation) in this region. This high pressure also acts on the combustion chamber, increasing the reaction force. 

In the turbine and nozzle, the variation in the cross section creates a surface area facing the rearward direction, where the gas pressure acts, resulting in an force applied in direction opposite to the compressor. The net result of all these forces gives the thrust.



Image from Aircraft Performance and Design by John Anderson; taken from quora.com