There are a few MSP objects that do exactly what you’re looking for- take a look at gizmo~ and freqshift~.
In order to create "harmony" you will need to have multiple channels- one with the unprocessed "C" (straight from audio input) and others with the pitch-shifted manipulated input (the "E" you’re going for).
I think the only thing you will find tricky is if you want the harmony to stay in a specific key or mode, rather than atonal, which is what the simplest arrangement would give you. It is definitely possible, just might take some experimenting.
One issue is always latency, but if you make sure to keep the fft size low and your I/O and signal vector sizes reasonable you should be fine.
Sorry I’m not sure what you mean about wavelength and buffer.
I’ll dig into freqshift and gizmo tonight and see how far I can get.
RE: wavelength and buffer – not worded clearly. I meant that shortening the wavelength to raise the pitch; but if I did this, then I would run out of sample and have a blank spot that would need to be filled in by something else.