Does your spare room get direct sunlight for most of the day? If so, place your
seed trays (&
seedlings) where the sun will shine right on them.
(DO NOT set your seedlings right next to the window where they'll get too hot when the heat is magnified by the glass. I learned that one the hard way!) I'm not sure about reflected sunlight (foil on the walls), as it might not be adequate for required photosynthesis.... but it's worth a try.
Healthy
plants need 3 essential things during all stages... energy (from sunlight), carbon dioxide (from moving air) & water (from the soil). Without adequate amounts of each, a
plant won't be able to reach its full potential. (Foliage, blossoms, fruit,
seeds.)
Heat is also important in germinating many
seeds.
(But not ALL seeds. Off the top of my head, I can think of a few that prefer cooler conditions, such as lettuce, carrots, peas & Broccoli - Cauliflower family.) While some
seeds don't seem to be fussy about temperature, there are those that insist on very warm soil before they'll sprout. (Basil & Pansies come to mind).
You can try starting
seeds in your spare bedroom, but once they sprout they will HAVE TO have adequate light, or they'll get spindly & just wither away.
From your picture, your mini
greenhouse looks like mine. Last winter I used mine to start the slower germinating
seeds (indoors), but when it was time to start the faster sprouting
seeds, I ran out of space & everything ended up on my garage shelves.
Using hooks that I made from a clothes hanger, I hung 24" full spectrum fluorescent lights above the shelves. With the plastic cover zipped up, it held the moisture in quite well & maintained a steady temperature, but there was no air movement... so I put my little fan inside. Since the shelves are open "racks", the air was able to move around nicely. (Remember that too much condensation inside that space is not a good thing, because the moisture creates a perfect environment for bacteria to breed, that will infect the
plants.)
You sound like you have some pretty good ideas, so dig in & give them a try! Be sure to keep a journal, so you can remember which ones are the most successful. I hope you'll share what worked (& what didn't work) for you... & keep taking pictures!