What is the mechanism by which reactants in endothermic reactions absorb energy from their environments?
In your first paragraph you state at the end that photons are released. In all reactions, except the rather few chemiluminescent ones, this is not the case. 

The energy difference is made up of vibrational and rotational energy of the products, which by collisions with solvent (or inert buffer gas) removes this energy.  This energy transfer occurs as the product is initially 'hot' with respect to the surroundings and thus energy flows out of this into the surroundings, heating it a little, until equilibrium is reached. In femtosecond laser experiments it is possible to observe the flow of energy when in solution and it is very rapid, not much longer than a few picoseconds. Ir photons could be emitted but the radiative lifetime of levels is much, much longer than collision lifetime so that i.r. emission yield will be tiny. 

In the low pressure gas phase (where collisions with buffer gas are absent) infra-red can be observed (called infra-red chemiluminescence as it comes from excited vibrational levels) in reactions such as $\ce{H + H2 \rightarrow  HF(v'J') + H} $ where $\ce{v'J'}$ are excited vibrational and rotational levels. Many experiments have been reported by Polanyi and co workers for which Polanyi received the Chemistry Nobel prize. This reference https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1986/polanyi-lecture.pdf gives a clear account of this important work 

In the reverse case it is the few molecules that by the Boltzmann distribution have enough energy to surmount the activation barrier that are important not promotion by ir photons.