That flex duct won't take the heat right off the plenium. Your going to have tempatures of over 200 degrees hitting that flex duct...
I did a quick google search for :http://www.google.com/search?hl=en&sa=X ... 27&spell=1http://www.rewci.com/4infldu25.html
Operating Temperature Range:
-20 Degrees Fahrenheit to 140 degrees Continuous
-20 Degrees Fahrenheit to 250 degrees Intermittent
You might end up with a melted mess, plastic smoke all though your house, or even worse a fire. Its not worth it. Get some big fat 8, 10 or 12 metal duct at lowes and screw it together. The duct, screws, tin snips might only be $50-$75. You don't need a cold air return to have a hot house, I'm proof of that. You can add that later.
Take a look at some of my posts
Mar 8 shows my setup good :http://nepacrossroads.com/viewtopic.php?t=1485
12/18/06 shows me getting started..http://nepacrossroads.com/viewtopic.php ... c&start=15
The bathroom can wait, it will only cost you alot of money. The coal furnace will save you money and can be done in the middle of the winter. When your forced to stay inside.
Good luck with the baby. I have two little ones myself. When they are first born they sleep alot, take advantage while you can.