Choice of Crankset

The new crankset is fine. The only difference between an "11-speed" crankset and a "9-speed" crankset is a slight difference in chainring width; you can use a narrower chainring with a wider chain.

However, it is very worrying that you had a crankarm fall off—twice. That isn't normal for a correctly set up bike. If you bought this bike new from the same shop that sold you the new cranks, they should have been falling all over themselves to make this right at no charge. And also, if so, it does not give me confidence that they installed the new cranks correctly.

Completely agree with what you are saying. To say I was upset and frustrated would be an understatement. But while the work from the shop has been shoddy, I always ask for compensation and they haven't objected, giving me the helmet and lock for free (~$170). They upgraded the crank for free and they will be swapping the whole bike out when they get a new shipment of them and I'll have them install the upgraded crank on that one when they give it to me, then get it checked out elsewhere just to be sure. The bike had a recall on the original crank but the above mentioned Shimano crank that came with mine was visually different than the original one on the specialized listing/recall. However, the recall was for the same exact issue with the left crank arm coming loose. Guess I'll have to wait and see how the RS510 fairs. It is definitely a much higher quality crank than the one I had but the left arm still attaches to the crank in the same fashion, bolted around the end rather than slip fitted over which makes me nervous. Thanks for responding, feel good about keeping it for now rather than having them swap it again. I Was just worried that they gave me something inappropriate/improper for my chain/rear setup.