I believe that one of the posible reasons you had to cut your derailleur is because of the increased chainline whern installing the mtn crankset on the bike. Road bikes are usually set up with a 45 mm chainline and mountain cranks with a 50 mm chainline. The usual method of putting mtn cranks on a road or touring bike is to use a shorter bottom bracket to maintain a 45-46 mm chainline. That is the reason a lot of us use square taper bottom brackets; there are more choices of bottom bracket length.
However, I think the primary reason for needing to cut the derailleur is a 42/32/24 crank combination. That derailleur will not take less than an 11 tooth difference between the big and middle chainrings without the backplate rubbing on the middle ring when shifting to the big gear. If you would have gone to a 44/32/22 it would have worked well, with a 12 tooth difference, and would not require cutting the derailleur.
However, you did get it to work
I've put this combination, 44/32/22 with an 11-34 cassette, on several bikes and it works well with both 105 and Tiagra shifters. IMO the Tiagra 4503 front derailleur is probably the most versatile FD out there. All this may not be too relevant because I'm basing my opinions on 9 speed setups, and I believe your setup is a 10 speed. I think I'd have fallen out of love with the Hollowtech bottom bracket or see if the large chainring could have been replaced with a 44 T.
