I am not sure what you report. You say "at a minimum left-sided colitis", but I wonder if you mean "minimal". Rectal meds work best on rectum and sigmoid colon, but can reach the left colon (especially the lower part of the left colon), so having left-sided colitis would indicate rectal meds. On the other hand if they are saying you have pancolitis, with mild left-sided ulceration, then I could understand why rectal meds would not help.
Beyond that, some GI are reluctant about rectal meds based on whether or not it was in their training: Read through this Q&A from a gastro journal:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2886486/
You might share a printout with your GI.
As for the retention issues you mention, Fleet enemas have chemistry designed to promote evacuation. It is much easier to retain something that is not a laxative.
Regarding the FT and the exact procedure, some FT is done during colonoscopy, some during flex-sig, some done with enemas (using either a bottle or a bag). There is ongoing study into whether enemas are as good as colonoscopy, or how many doses and for how long it might be needed.
Where is this clinic that is encouraging FT? Most people report their GI does not want to try FT.