I just had a similar ordeal with my xf622..

I had to replace the lithium and re-tension the clips that held the fuse in place, as the fuse was not held tight enough to make sure contact.

Also my main battery had a damaged lead end, where it socketed into the connector in the radio, the contact on the female part at the battery wire end was not making contact with the pin. A re-bend with the pocket knife and that was fixed.

The thing still would not come on after all that, and after verifying voltage was getting to the main boards.

To re-set the poor confused processor, remove the main battery and (ideally the lithum cell would not be in place for this, but in my case it was and it worked fine) using a small sheet of aluminum (aluminium in UK?) foil, short all the pins on the main board (by 'main board' I mean the one with the processor and the lithium cell). This pulls all to earth and also shorts out the lithum cell for a few seconds, but it can deal with it, those things don't put out much current anyway.

In the case of my 622, i only shorted the leads of the main processor all together by pressing the foil over it and running my fingernail over the foil along the sides where the leads come out of the black plastic body of the IC.

I used to work as a factory authorised repair tech for sony, pioneer, alpine, etc... and sometimes this was the only way to reset some of the many things that got confused after the lithium cell was replaced..

Stick the main battery back and see if it comes up. You may need to try more than once...

Be mindful of static electricity when fooling around with modern printed circuitry, as a small spark will fully ruin a cmos chip.

You mentioning the blown main fuse does scare me though.. hopefully that was due to your main battery being shot, but there may be a regulator or capacitor dead in there somewhere..