No, there is really no "good" external switch for this purpose, because the signal voltage from a cartridge, even a high output one, is very low compared to line level signals, and passing that signal through a switch will inevitably do harm. Some similarly recommend a Y-adapter placed in the line from cartridge to phono input. That's not a good idea, because of the impedance anomalies that will result. (It's too complicated to explain here.) However, it might "work" to place a Y-adapter in the line between the phono stage output and the linestage input, if you are using separates. Because impedances will also be affected in this topology, you'd have to experiment on a case by case basis (phono stage/linestage combos).
Depends upon the design and quality of the switch.
I don't understand the comment that the DV17D3 per se isn't good for mono compared to other modern stereo MC cartridges. All modern stereo cartridges should be about the same in mono, for good or ill, and assuming a stylus shape that is "modern", i.e., line contact or modified elliptical type. A stereo cartridge with a conical tip might do mono slightly better than the foregoing, but such a stylus shape is not very good for stereo in the first place.
Finally, you didn't ask this question, but I wanted to comment that having a mono switch on your preamplifier to my ears gives about the same (good) result as using a mono cartridge. I have both, and that's my opinion. This thread will go on and on, so I would also mention here that there are both "true" mono cartridges and mono cartridges that are made from stereo cartridges by bridging the channels internally. In the latter case, that is more or less like using a mono switch. Also, IMO, there is tremendous benefit to playing mono LPs in mono, no matter how you get it done. On the other hand, most mono LPs also sound "good" in stereo mode, if you cannot afford the equipment needed to go mono. You'll just hear more noise due to dirt in the grooves and to recorded hf that amounts to noise and is cancelled out in mono mode.
Depends upon the design and quality of the switch.
I don't understand the comment that the DV17D3 per se isn't good for mono compared to other modern stereo MC cartridges. All modern stereo cartridges should be about the same in mono, for good or ill, and assuming a stylus shape that is "modern", i.e., line contact or modified elliptical type. A stereo cartridge with a conical tip might do mono slightly better than the foregoing, but such a stylus shape is not very good for stereo in the first place.
Finally, you didn't ask this question, but I wanted to comment that having a mono switch on your preamplifier to my ears gives about the same (good) result as using a mono cartridge. I have both, and that's my opinion. This thread will go on and on, so I would also mention here that there are both "true" mono cartridges and mono cartridges that are made from stereo cartridges by bridging the channels internally. In the latter case, that is more or less like using a mono switch. Also, IMO, there is tremendous benefit to playing mono LPs in mono, no matter how you get it done. On the other hand, most mono LPs also sound "good" in stereo mode, if you cannot afford the equipment needed to go mono. You'll just hear more noise due to dirt in the grooves and to recorded hf that amounts to noise and is cancelled out in mono mode.