1) If you think your amp may be failing, it may be a good idea to just have the battery replaced so you do not need to worry about replacing the sounds at a gig.  I have never tried this, but if the internal memory cannot be saved, I doubt that loading new patches would actually work.

  2) Unfortunately Line 6 has no info on this topic.  Perhaps another end user can chime in with their experience.

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Its been awhile, but I got mine from Digikey

1P135-ND Battery, Lithium coin $1.48

I think Line 6 support sent me some in the past also.  These things last for 10+ years, so while a slot style would be nice, I figured I wouldn't be changing it again, so I just de-soldered and re-soldered with the replacement.  Could probably find a coin type slot with equivalent values though and that would have been a better factory default than what they used, but there's also the vibration factor and maybe the thinking was that a slot type was more prone to falling out.

Here're the diagnostic tests:

Hold the Save button while turing the amp on until SrA is displayed.  You'll get a PAS or Err msg for each check.  Bank UP and DOWN moves between the tests, TUNER button engages the tests.  The tests are:

SrA - checks the SRAM, problem with it involves U15

EPr - EPROM (U14)

LEd

bAt

bUt - to exit press functions 5&6 together

PEd - need proprietary test fixture to perform this - skip it

AC - problem is R38 or Q1

idi - tests Midi - need wrap back cable

AUd - problems involve U9, U12, U13 or U19 or the DSPs

AdA - need a voltage divider to complete the test.

Battery is a Lithium coin - Digikey 1P135-ND  Ask L6 for this as I've gottne close but not cigar from Digikey and Mouser in the past, alos have gotten the right ones in the past.  L6 should have them and be able to send them free.

Hope this helps.