So there is a provision that durring a battle you can use the top card from your combat deck. I have never used this provision but I would think that there would be the possibility that a reinforcement card could be drawn. Since there are no combat values on those cards, I would assume automatic loss. But you can't play a reinforcement card without a combat card but that could be the intended risk be randomly drawing the top card?

This option is a standard rule for every game, it's always available. But when you draw a reinforcement card this way, you have to discard it and draw another card, until you get non-reinforcement card (containing combat values for units, which can be used to resolve a skirmish).

By playing Combat card directly from a deck, you can't add any reinforcement card to a skirmish.