If you make a call with the speaker on, both microphones will probably be used – primary microphone (the 1 mm hole at the lower end of the phone) and secondary microphone (the 1 mm hole at the top end). When you call the old-fashioned way (display at cheek), most likely the primary microphone is the only one used. So it might still be damaged.
Fairphone OS comes with Checkup, a set of hardware tests (including microphone tests) for self-diagnosis. LineageOS comes without Checkup pre-installed, but you can still install it as an app (it is available in the F-Droid app store). I recommend to do so so you can test your hardware. Alternatively, you can just try to record your voice using the primary microphone with a simple sound recording app – and play back the recording then.
P.S.: Please clarify if you are using an FP2 or an FP1. The category (Help FP2) and your user profile are giving contradictory information. My suggestions assume you are now using an FP2.
Most likely, yes. However, you can still give screwing the current bottom module out and back into your FP2 a try to see if it helps (not very likely, but you will need the screwdriver Phillips #0anyway if you get a new bottom module – so it doesn’t hurt to give it a try now already)
Tried this and found some corrosion on the connections. Cleaning didn’t solve the issue, so a new bottom module is now on the way. Thanks for the help @urs_lesse !