The long answer: I do not get it... I am totally confused by your question. Let's see.
As I am attempting on printing a photo-magazine in black and white.
So, just use one ink. Black. If you really want deep black, you should ask the provider to use a deep black ink. In very rare occasions, you can ask the printer to pass twice each paper with the exact same plate and alignment. This will give you a deep black k200. Yeap it sounds weird, but it works. The point is that the printer (and you) needs to be willing to experiment.
I am losing rich blacks in my photos when converting them to CMYK
Did you want black and white or not?
CMYK won't appear as black as RGB blacks viewed on a monitor.
You should have 1) Your monitor calibrated and 2) The CMYK simulation turned on even if you are working on RGB.
He also mentions how a proof print would be a much more accurate representation of what the preview in ID shows.
Depends on if the proof is really a proof and not only a simple print. This should be (the same as your monitor) calibrated. Did I mention the word calibrated?
Still, I do fancy a bit of certainty or control, so I went into Photoshop to play with the photos a little.
If you do not know what are you doing, do not play with a CMYK file. Do whatever on an RGB one, but leave the CMYK alone.
When converting the photo to a CMYK color profile
Ok. You did work on the RGB file. Good.
You do not convert a RGB photo to "a CMYK profile" you convert a photo on RGB model to CMYK model using a specific color profile.
If you choose your color profile correctly you do not need to worry. Leave the program do the conversion.
Darkest blacks in the photo. These were approx. 90, 80, 60, and 100 respectively.
Ouch... something is not right here.
There are some color profiles that use 330% TAC (Total ink) this is the case of Fogra 39... do you need Fogra 39? If your provider is really ok with the TAC, go ahead.
But I am wondering if you are just rounding numbers (c91 m79 y62 k98) or tweaking the file.
Modern printers can use Fogra39, new inks, additives, special drying systems, conditioned and humidity controlled air and environment. But if your printer does not use it try to stick with a color profile that stays on the 300% TAC. Ask them.
This is in total much more than the 300% which I have been told would give drying issues, and thus smudges.
Depends on the printer.
Are these values accurate?
They are not, I wrote the accurate value above)
Should I lower these blacks, even though the photo previewed in ID already seems quite dull?
No. If you want (or need) lower values, use a different color profile.
An image well prepared, well printed on a good paper using a 300% TAC will look very good. The 330% is just a bit deeper, but only if you put them side by side. A normal user will not say. "Oh, this photo of the universe is dull, they should have used more TAC".
If you want to punch the colors a bit, try to use a varnish.
Remember that color profiles are for specific combinations of paper, inks, and printing. But the main factor is the paper.
The 300% and beyond TAC is for coated paper. There are some papers that use even less TAC, like 220 ish, so. Ask the printer.
The short version... Use another color profile. Not Fogra 39.