It is perfectly safe to mount rack and mudguards onto one eyelet. My commuter bike is like this, I have hauled heavy shopping and used the bike for many years.
Fit a long stainless steel allen bole of the correct length. I prefer mine to protrude about 1-2 threads. Fit the rack first then the mudguard stay then a washer. You can add a star anti-rotation washer if you want.
Any bikeshop or hardware shop should have the correct allen bolt.