The tablets last a LONG time in the bucket and you can use them while on vacation/etc ... you just have to KNOW that your CYA will go up ... so those could be useful in the next few years. Not sure what to do about the Nature 2.
Any FC is always fighting algae, the recommended shock level is to be positive that you stay ahead of its growth. Getting a little low may not hurt, but it certainly is not helping.
The sun helps break down the CCs and since you still show them, I would think leaving the cover off is best.
But, I think you should be OK to turn the heater on ... although you should not swim with FC > shock value.
Any FC is always fighting algae, the recommended shock level is to be positive that you stay ahead of its growth. Getting a little low may not hurt, but it certainly is not helping.
The sun helps break down the CCs and since you still show them, I would think leaving the cover off is best.
But, I think you should be OK to turn the heater on ... although you should not swim with FC > shock value.