The reason to include the barley is to bulk up the filling and use less meat:
all the flavours of a proper beef stew, while the barley reduces the amount of beef needed while still retaining all the flavour
If you want to stay close to the recipe (and it's up to you whether you want to or not), you probably want to use something that retains a bit of a bite and doesn't disintegrate into your sauce. Instead of barley, most whole grains should work, from wheat berries to rice, and I would pick wholegrain rice over white, given the choice. Considering that you need only 100g, you could - if you are really determined - ask your local bakery for a cup of whole grains, they might have wheat or spelt berries for some of their breads, or check an organic store if your local supermarket is too limited.
Lentils are always mushier, because they have a soft center in a thicker skin. Once that breaks, the lentil is prone to disintegrate. You can mitigate that a bit if you cook them with salt (contrary to the often-repeated advice to avoid salt). Flavor-wise, lentils are a common choice as meat substitute, so while you wouldn't be that close to the original, the results should still be good. Choose a type of lentils that is used for salads - so black and green (verte de puy) sound good. They will darken your sauce a bit, but that's fine considering the given ingredients. Do not precook them, if you are going to simmer them quite a while in the sauce as per the recipe - or if you do, only very briefly, five to ten minutes perhaps. You may want to adjust the lentils-to-liquid ratio though. The rule of thumb is one part lentils to two parts of water, while barley needs between 2.5 and 3.5 parts, so 150g lentils plus 550g water should give you roughly the same consistency as 100g barley + 350ml (absorbed) +250ml (leftover for the sauce) water. But that's just a ballpark number, cooking time and evaporation will influence this, so use common (cooking) sense and aim for the usual cottage pie filling consistency.