Identifying Indels from Chromatograms
If you want to call heterozygous SNPs and INDELs, you may try polyphred. I believe there are a couple of other similar tools, but I have forgotten their names.


  How can evidence of (het) indels like this be distinguished from noise in the chromatogram?


Your two points are correct. Basically, at some position, you start to see two streaks of overlapping peaks. Once identified, the two streaks can be aligned to each other if you shift one of them. This is a strong constraint, but it is hard to tell by eye because it is not obvious how to assign peaks to streaks without running an automatic tool. Doing this is even more difficult given noisy traces like your last figure. I think you'd better run polyphred or similar.


  the putative indel seems to start in a homopolymer region—cast doubt on this?


The INDEL mutation rate in long homopolymer runs is orders of magnitude higher than elsewhere in the genome. This has two implications: 1) you expect to see more homopolymer indels; 2) PCR polymerase is more likely to introduce errors in homopolymers––a higher fraction of homopolymer indels tend to be wrong.