Pay 1000 Life Points. Special Summon 1 Level 5 or lower Fusion Monster from your Fusion Deck. (This Special Summon is treated as a Fusion Summon). The Fusion Monster Special Summoned by this effect cannot attack, and is destroyed during the End Phase. Only 1 "Instant Fusion" can be activated per turn.

If I use this on reaper of the nightmare then sacrafice it and book of life it in the same turn is it basically reset and free of the limitations of instant fusion?

If it leaves the field in any way and is brought back, yes, it will be able to attack and it will not be destroyed at the end of the turn.

I'm not sure if flipping it face-down will remove the restrictions as well. Netrep is broke'd at the moment, and I'm too lazy to use UDE's website. If it's a pre-existing condition for the summon, then it'll keep the restrictions even if flipped face-down (which I think is how it's ruled). But if it's considered an effect of Instant Fusion, then flipping it face-down would reset those limitations. You'll have to wait for someone else to give a definite answer though.

I asked the exact same questions at CoG (generic fusion monster LVL 5 or less) and got pointed to the Return from the Different Dimension ruling:

So, unless BKSS is in effect, it would not be sent to the GY.