I am a proud owner of what seems to be a 30 year old or so Serissa Japonica
in slight semi-cascade / slanting style.

I won't comment too much on the age of the tree, but what makes you think it is
30 years old? What is the trunk caliper? The correct name is Serissa foetida.
The species originated in China.

Currently the tree is in a plastic pot with a good amount of healthy white
roots wrapped all around. The root mass does not seem to be too overcrowded
yet. The tree is about 15" tall and it lives in a 10" pot. Upon watering the
tree it took a good amount of time for the water to appear in the drainage
tray.

From your description, I would say the pot is about the right size, but the
tree is badly in need of repotting in a better draining soil.

I plan to mist the tree in the morning for added humidity.

This will not help significantly. You need a humidity gauge, and possibly a
humidifier.

inside it will stay until I buy a house. I do have ample amount of
sunlight in the apartment. But I have read conflicting opinions on the type of
sunlight the Serissa desires. Most seem to suggest that it likes medium and
indirect sunlight.

You should try to make arrangements to keep the tree outdoors in the summer.
Outdoors, Serissa likes just a little shade. Indoors it should get full sun.
Under fluorescent lights is even better. What is more important, try to keep it
cool.

Does anyone residing in the Northeastern part of US have experience with
this tree?

I have killed more than my share.

would it be safe to transplant and root-prune the tree in a few weeks?

Wait until the weather is warmer & it is acclimated to your conditions.

Oh, and how bad is the "wretched" smell produced by root-pruning?

It smells like a slightly spoiled radish. I have smelled worse.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)