Fuses act on rms current over certain period of time, so I think in this case 500mA-rated fuse working @ 495mA-rms current may, or may not blow, that will vary from fuse to fuse as the current is just to close to the fuse’s rating ..
You’d better select a fuse that is rated one size up, that is 630mA, and this one should work fine ..
For a 100% ampere rating the datasheet guarantees a minimum opening time of 4 hours only. And as IanP has pointed out, the nominal melting is defined by I2t (A2sec), so, it the RMS current that counts. So, it is better going for a higher rated fuse.
Yes, electromigration will happen. Current and temperature
accelerated. Your manufacturer ought to have reliability
data of this sort available, though you might have to dig
or ask nicely.