Not having a green house but having had to protect tender plants over winter more times than I can count, I'll offer the following suggestions:
Add another layer of clear plastic over and around your greenhouse (if it's not too big to do so). The small but effective layer of air you'll create will lessen/slow the escape of heat from the glass panes. Big box stores carry rolls of heavy gauge plastic which open out to large widths and work perfectly for the purpose. Use duct tape or similar to attach as necessary. Remember that heat radiates easily through single pane glass (both in and out) and any efforts to minimize that heat loss will pay off over both the short and long term. In summer, just remove the plastic and fold for use in future seasons...
The size of the extension cord you use may not necessarily provide more heat but may/will reduce your electric bill if you have a long run (distance) from the power source to the heater. Heavier is always better once you get past the "just light bulbs only" stage...
Although it may not be possible on short notice, add dark colored bricks or masonry blocks (used or broken works fine) to your greenhouse floor and shelves to trap ambient heat (both sun and other sources) during the day and which will be released slowly during the night time hours.
I doubt that the Christmas lights will add even a degree to the overall temp although I applaud you for thinking of possible remedies.
Good luck!
Steve

I'm not sure at what temps the tropicals would be damaged. Hopefully someone else will. Maybe check in with the tropical plant forum for that information. I wonder if bunching your plants together and using a couple heat lamps shining on them in addition to the electric heater would help in the short term. If so you'd need a heavier cord as Steve suggests.

my tropical are usually OK until they actually freeze. I have the bougies up high to get benefit from heat rising. I have the most tender plants near the heater and tougher plants on the edges. I used to keep a big storage tub full of water to help hold in heat. Don't know if it helped, but kept it on the N/w corner. Stopped doing that a few years ago...not sure why. We now have a metal roof so the GH doesn't get very hot. We use those electric heaters that look like old radiators but have oil inside. They are plugged in using outdoor extension cords from the house.

Some things make it some do not. sorry.....c

Well, so far things are okay, it got down to 40 again last night. But warmed up well today. I think the nights are going to be warmer for the next week. Thanks for all the input.

On the greenhouse forum they had some great suggestions, dh was planning to fix things this weekend but now has a coast fishing trip planned with dear sons who are coming in for the first time in a while, from college/work. Maybe it will get done before the next cold front?

Your ficus will probably lose some leaves just from the moving alone, but it will be fine. The rubber tree will probably recover too. Don't know anything about the others. My goal is to just keep everything above freezing and most everything survives.

I have heard that bubble wrap is a great thing to use a second layer under your plastic. I save everything that comes in the mail and have some big pieces that That have wrapped big stuff. I don't have a GH but I wrap plants with it.

Well, my DH has installed a wood stove in my greenhouse! Gotta love that guy. It was still so warm tonight, we haven't done a test burn yet, I suspect that it will cool off enough tomorrow to do a test burn and make sure it doesn't need adjusting! So relieved!

I just got my second novel published to Kindle, life is good!

So--IT'S COLD NOW, keeping two wood stoves burning all day and the greenhouse all night last night is tiresome, but I only went out at 11:00PM and a little after 4am. Not terrible, and dh stoked it up well before he left this morning. He has several large tubs of wood for the greenhouse and the house each. Can't brag on him enough!