I appologize for the delay in my reply and i did receive your email, however, there are many factors involved so before offering you my opinion I took the time to confer with a friend who is a masonary contractor.
Before we examine your problem we need to consider the basic structure of concrete itself. In its simplest form concrete is a mixture of cement and gravel. If left to its own devices when concrete is poured into a form it will produce a very porous surface finish full of nulls and voids. During the pouring process a huge vibrator is inserted into the form to shake the mix, causing the gravel to settle and causing the liquid portion of the mix (the grout) to settle against the form walls which will produce a nice smooth finish. Basically the same thing takes place when concrete flatwork is finished. They use a device called a "jitterbug" that looks similar to a elderly persons walker with an expanded metal plate on the base to tamp the surface forcing the gravel down and leaving a thin layer of the grout on top that can be finished smooth.
While the external grout layer will form to a nice finish, it is rather brittle by comparison to the actual concrete mix.
The problem you are encountering is most likely the result of one of two problems.
Often when concrete is poured in forms, if the mix is not adequately vibrated there are numerous voids in the surface. When the forms are removed the finishers then trowel in a mortar mix and trowel it smooth to fill the voids. It is quite possible that the loose concrete you are seeing is some of those patches that are coming loose, if so, it has very little effect on the overall integrity of the wall.
It is also possible that the areas that are crumbling are natural voids in the rocks in the concrete which filled with the liquid grout when the concrete was poured and vibrated. Under extreme conditions of temperature and moisture those areas of grout that lack the gravel can flake off leaving a small depression in the wall such as you describe.
If the effected areas are small, 2 to 5 inches in diameter and the depths is only an inch or so, it should present no structural problem to the wall.
To repair those holes I would begin with a masonary brick hammer and chip away as much of the loose material as possible, then wet the concrete with a brush or wisk broom saturated in plain water to remove dust and prewet the concrete.
You can then brush on a thin layer of latex binder (available at a masonary supply yard) and finally use a wooded float trowel to fill the voids with mortar mix and trowel it smooth.
Given that you intend to leave the structure unattended with minimal heat through the winter I think i would wait until late spring to make the repairs.
A bit of concrete Trivia- According the the information i recieved from my friend the mason, concrete reaches 90% hardness in about 24 hours. It continues curing and reaches about 95% maximum hardness in 21 days, which is why they take core samples and wait 21 days to test them. It then takes about 99 years for concrete to reach its maximum hardness, then it stays stable about 25 to 50 years, then it begins to deteriorate at a slow pace.