Adding external bluetooth antenna with U.FL connector
The more &quot;different from 50 ohms&quot; the trace impedance is, the more power will be reflected at the impedance mismatch boundary. That can result in:

less power being transmitted usefully
less power being received usefully
damage to your transceiver
spurious emissions (i.e. FCC certification problems)

I would generally avoid putting any traces near the antenna trace if it can be avoided, but especially digital traces with fast edge rates as they could adversely impact the SNR.
If you've got a 4-layer board, I like to use a coplanar microstrip waveguide. There are lots of calculators available online for such a thing, for instance: https://www.changpuak.ch/electronics/Coplanar_Waveguide_Calculator.php. You just need to look up the stack-up and dielectric parameters from your PCB fabrication house and plug in numbers so that you get near 50 ohms. Just make sure you have nice solid ground planes around and below the antenna trace and you'll be fine.
Pay attention to the layout recommendations in the u.Fl connector datasheet too. They almost certainly will inform you about what the ground plane situation should be around and below the connector.