If you currently have a 30-42-52, the BCD (bolt circle diameter) of the outer two is almost certainly 130mm, and the BCD of the little one is 74mm. The smallest you can go with 130 is 38 teeth, meaning you cannot put a 32T in the middle position; but the 74 can go down to 24T. 24/34 will give you your 19" gear. I don't think you'll be sorry you went so low. We have 24-42-52 on our tandem since my wife is anything but a good climber, and we have done many-miles-long 10% grades. It shifts fine. I do recommend keeping the middle ring bigger than the average of the other two, for better shifting. IOW, if you have 24-XX-52, the "XX" should be at least 39. It will also be best to keep middle and outer rings that are mated to each other, ramped, and pinned. (They will be all three if you keep your current crankset and only change the granny ring. The new granny won't be mated to the middle ring, but that's life.)
To keep proper mating between the two biggest rings if you change the 42 (which is a B type) to a 39 (which is an A type), you should also change the 52 from a B type to an A type. Correct mating makes sure that the teeth of the two rings and the ramps and pins are aligned such that when you shift up, the chain gets caught and helped up at just the right place so the rollers will go between the teeth of the bigger ring instead of trying to ride on top of the teeth and more easily drop the chain off the outside of the crank. Properly mated rings make for very quick, trouble-free shifts.
The small MTB crankset will work with your BB, but not with your front derailleur, especially if it is a braze-on type instead of clamp-on, since it won't go low enough on the seat tube.
You can put a 13-34 cassette on, to get the low end, but you'll have to change the rear derailleur to a MTB type to handle the big cogs. You will want a normal-high type, instead of normal-low or RapidRise which has recently become common in the MTB world. Normal high means the derailleur will go out to smaller cogs when you release the cable.