I can see problems with your approach.
Replacing tuning pegs can be a problem, especially if the holes are munged up by the process. Check 'em carefully, and perhaps run them a turn or two further into the wood, if you feel any looseness.
Using guitar strings or even piano strings with windings that go over the edges ("saddles" or "nuts" in guitar terms) give you a different sound than standard autoharp strings, which, if you notice, have the windings stripped off for the contact points. The difference can be very noticeable, and not so good. If you can't strip the windings from guitar strings (and it's a tricky technique, for sure), use proper autoharp strings. Buy 'em in single lots, so you can double the ones you want to double.
And replace the first two unwound notes with wound strings (they will take the extra tension). I recommend this on all autoharps, myself, even if not using odd-ball tunings and stringing patterns.
Good luck. I have no recommendations as to double-stringing or tunings -- I tend to use 'em as they come, though I did once take an old black A model 12-bar and re-do the chords so I could play in F/C/G/D/A/&E, plus four relative-minor chords. I did this by totally eliminating all 7th chords. The result is I could play lots of raggy tunes with circles of fifths and such, but couldn't play "Skip to My Lou." Ah, well.