I suffered the same with this. In the end I sat the Mac Mini on it's end and lowered the originally fitted hard drive into the pin lugs on the casing. Once the logic board was in place it stopped it from falling forward / out of position.
Deconstruction guide good (with exception of having to remove fan screw in step 4).
Rebuild: I was surprised how quickly I forgot how bits fitted together! ENSURE iFixit supplied cable goes BETWEEN hard drives!!! Step 36 was easier once I'd tipped MacMini on it's front & lowered the original hard drive into the lug holes in the casing (easier than pushing it in flat).
The cable that supplies the board with power (black multi-strand - next to PSU) got caught up between hard drives and the PSU making logic board hard to fit. Once I'd pushed the cable down between the drives with spudger all OK.
Finally I had a moment of panic when I thought logic board was misaligned. The black plastic cowel (semi circular piece with single screw) didn't seem to align! Push it just a bit further & it's fine, but I ended up taking it apart again to find this out!!
A fairly simple procedure taking me about 45 minutes, so thanks iFixit!
TIP: Take your own detailed macro photos to refer back to if you're uncertain!
Same for me, couldn't get it to move without feeling it was going to break so I just removed the screw.