You know how you turn on an electric heater and the filament begins to glow? That is energy being converted to light, so not 100% efficient.

Not quite. Some is lost in magnetic flux and mechanical deformation. But that is a VERY small about.

Yes, but, it is also possible to achieve greater than 100% efficiency in using electrical energy for heat. You can use electricity to move heat from where you don't need it to where you want it. The amount of heat energy you move can be greater than the amount of electrical energy put into the system, so it's greater than 100% electrically efficient. It's well below 100% thermal efficiency, of course.

Technically, yes. Even the internal resistances outside of the heating elements eventually radiate into the space. Since the purpose is space heating, it's not a waste product and they can be roughly considered 100% efficient.

The reason heat pumps are more efficient (i.e. around 300% or more) is not that they create more heat from the same amount of energy but because they concentrate and move existing heat from one source to another.

Yes, but a heat pump for heating is somwhere from 200% to 500% efficient.

Yes, except a heat pump is capable of being more than 100% efficient because the using the power to move heat around is more efficient than converting power directly to heat

Essentially all electrical devices are, in addition to whatever else they do, also basically 100% efficient space heaters. A PC running on 300 watts is doing things with that 300 watts but it all ends up as heat, the vast majority of which stays in the room. A light bulb puts out light, but little of that light leaves the house, it's all getting reflected and absorbed until it's mostly a heater in your house.

Heaters sometimes produce a little light or sound, so not 100%, but very close.

There's an interesting aspect of this that I have not seen mentioned yet. While this is true you are usually better off using your residential heater rather than an electric space heater because residential heaters are frequently over 100% efficient. That is, they deliver more heat for the energy expenditure than if you had converted the energy directly by redirecting ambient heat. Heat pumps are this same principle taken to the extreme.

But you can heat an area with 'better than 100% efficiency' if you use a heat pump and move heat from one place to another. Coefficients of performance (cop) of about 2.5 I think are pretty common, meaning if you put 100 watts into moving heat, the area will get 250 watts warmer.

Only in the sense of releasing heat into the surrounding environment. But for instance, an electric boiler is not 100% efficient because not all of the heat goes to the water. The heat that doesn't go to the intended recipient of the heat is treated as loss.

tl;dr, yes If you want to watch a 20 minute video about it instead of 1 word answers, https://youtu.be/V-jmSjy2ArM

Yes it does. There's no such thing as an inefficient electric heater. I'm the US they sell 2 kinds. 750w and 1500w. The differences between them are all a matter of form factor. Maybe a nice fan to circulate might be what you like though.

I so want this to be true, but dont they produce radio waves?

Yes. The end form of all energy is heat.

The problem is that heat is a high-entropy source of energy. As a result, the losses come when you try to convert all that delicious heat back into electricity (like with a steam turbine). The "efficiencies" only go one way (and I put efficiencies in quotes because, as you pointed out, getting energy into the form of heat is inevitable, whether that's the form you wanted or not)