Extending the life of a cheap aluminium greenhouse

Covering with polytunnel plastic would work. You would need to buy insulation tape (it a foam backed tape, all tunnel companies have it). It stop the sun heating the metal work though the plastic when it’s tightly pulled over it. If you don’t use it the plastic will just melt. And it’d also stop the greenhouse metal work from piercing the plastic. As greenhouses are much more pointy than tunnels. Also you would need to secure it very well at ground level to stop it blowing off.

Fungus gnats cure , layer of vermiculite on top of all trays and pots.

Thanks so much, SC. That's very encouraging. I've been looking online for people doing this and haven't had much luck, so I thought it might be too impractical. I feel hopeful about the idea again.

I have some volcanic rock stuff I put in my indoor plants for the gnats but haven't tried similar for the seedlings. Can they push through if the vermiculite is applied at the time of sowing? 

Thanks again for the encouraging words.