I assume that racking canes are made of neutral, food- safe plastic that will not affect my beer. Although it is charming that you are so concerned about my melted plastic, plastic is still just plastic when it is melted and re-solidified. It does not transmogrify into something else unless you burn it.
What temp do you think a propane flame burns at?
Do you think that 'by feel' you can guarantee not exceeding any temp thresholds for smell/flavor creation or transmogrification?
As for the food 'safieness' of a racking cane, you would have to consult their NSF report for permitted type of contact and time of exposure.
Using a corny dip tune is a good idea. Maybe i can get one at my LHBS. However my budget is very small and i do not like to buy something if i can make do with crap i have lying around here.
Prices for corny kegs parts are obscene. Cheapest I have seen a dip tube is $5, and they are usually $10+. The vendor who makes custom thermowells and dip tubes might be able to sell you a 'combo' (croctopus?) model for <$10. It only involves adding a flange to a well. The tubing alone will cost you $4 at a metal shop.
You are correct that i don't think it's important to seal off primary fermenters. I usually transfer to a keg for lagering or aging or else simply bottle from the primary.
The protective CO2 blanket only has so much magic, and it disappears quickly after ferm slows down- what with diffusion, drafts, expansion/contraction, and what not.
If you are fermenting in cornies, a better system would be to put the thermowell through the lid, and if you make it airtight, maybe even with a post fitting, you can push your beer with CO2 to another keg. You could also just replace the probe/well with a real dip tube after ferm, and do the same thing. I just prefer to keep stuff sealed, plus I seal the last part of the ferm to build some carb to help push and shorten carb time.