Re: 2 layer layout help
"Controlled impedance" means that YOU (the board designer) are controlling the impedance of specific traces in order to ensure signal integrity. "Uncontrolled impedance" basically means you don't care about the impedance because the signal conditions don't warrant that concern.
All traces on a PCB are transmission lines. For critical signals above the audio range, the transmission line impedance should match the terminating impedance to ensure maximum energy transfer to the load. When there is an impedance mismatch, some energy is reflected from the point of mismatch. That reflected energy causes signal distortion.
An engineer designing a board will select the trace width, trace thickness, dielectric material, and distance to the return path to give the desired impedance. The factors affecting impedance are: trace width, trace thickness, trace cross sectional shape, distance between the trace and return path, and Er (dielectric permitivity - sometimes expressed as loss factor). The board fabricator will advise the desginer of any changes required to control impedance because of the Er for the particular batch of material they are using to make the board. They are also in the best position to know how their etching process will affect the cross sectional shape of the trace.
For low frequencies (DC-audio), the trace impedance is not a significant factor for signal integrity. Inductive coupling, capacitive coupling, and intermodulation are the greatest concerns.
With regard to your design - you are correct that the high current paths for the ultrasonic device should be kept separate from the other circuitry. Your PIC is a digital device operating at radio frequency (RF). The clock, and digital I/O are susceptible to false triggering from digital signal distortion that might be caused by inadvertently coupling a portion of the ultrasonic signal into a digital pathway.
The copper pouring I suggested is still a good idea - you just need to adjust the pour around the digital pathways, and your high current signal paths, such that they keep the two different types of ground returns separated.