How can I tightly join two perpendicular pieces of wood in a loft bed from the inside corner without access to the outer side?
I'd be cautious with anything holding up the top level that's not a bolt going all the way through the wood. The Ikea stuff tends not be overbuilt at all, so the wouldn't be a lot of extra wood to screw into to get a good hold. The extra braces are fine, but if it was my kid sleeping in the bed, I wouldn't want to rely only on some screws in &lt;= 1" thick particle board.



My gut reaction is this can probably be built largely as-designed in an 80" room (so long as the other dimension is at least as wide, anyway), though without actually seeing it in person it's hard to say for sure.

Here's how I'd do it, and for reference, I'll label the corners A,B,C,D:




Throw away the instructions. Or at least, don't follow them in order.
Build the longest edges first -- A--D and B--C -- by building them at an angle in the room. Basically any screw that go into the narrow sides need to be installed first.
You'll also have to assemble the piece you showed from the instructions running across the middle of the bed, even though the cross-braces it's screwing into aren't attached yet.
Now, with the frame in the middle of the room, do the remaining connections, attaching the A--D and B--C pieces together. 
Push the bed back against the wall




I have no doubt it'll involve some head scratching, coercion tools (aka hammers/prybars/etc) and some swearing, but it's a relatively simple design so it should be possible. 

Good luck!