Can you use bicycle boulevards for effective interval training? Using commutes for training can be a time saving idea. Let's say biking to work takes you 5 to 10 hours a week. That's plenty of time for exercising. Why not use that time to actually get fit which has many benefits?

Intervals are usually required for if you're looking into breaking the plateau. With frequent stops, maybe power meters make staying in our target zone easier.

If your only options are bike boulevards, greenways, trails, and low volume residential streets, can you effectively train? Safety is important. There's curbs, roundabouts, stop signs, yield signs, downhills, major street crossing, etc which can cause interruptions.

The intervals include anaerobic, VO2 max, threshold, and sweet spot. Some workouts say to do 20 minute threshold or sweet spot intervals, and others say to do VO2 max for a few minutes.

I'm doing sweet spot base training, and I plan on doing general build.

How much interruption is too long? Can I just aim for 40 minutes in the target power zones and for the rest of the ride recovery zone? It's likely to be interrupted 20 times by major street crossings. Each intervals might last 2 minutes followed by half a minute of waiting for the lights to change. Maybe they can be used for mini recoveries.

Can workouts also be split? Say your commutes are shorter or you're busy. You might split your workout throughout two or more rides or days, or use your breaks at work or school for your workout. For example, if you 4x10 or 6x10 intervals can you do 2x10 in the morning and 2x10 in the afternoon, or 3x10 on Monday and 3x10 on Tuesday? It's commonly agreed that you can split workouts without compromising health benefits but would you hinder gains in endurance or rely more on glycogen?
In principle yes, but keep in mind that, while you will be using public places you will be likely meet other cyclists who are not there to train and will act, to your eyes, just like moving obstacles: kids riding and suddenly stopping or turning, flocks of teens looking at their phones while riding one hand and then suddenly stopping their bike across the lane, elderly people just taking a ride to enjoy the sun, etc. 

You, while busy in training, might even call them like CSO (Cyclist Shaped Object), but in reality they are as legitimate users as you are. And I know fellow amateur cyclists who prefer to practice peloton techniques on the highway rather than on a cycling lane (dumb, isn't it?).

With that in mind, how heavily will that affect your training schedule? If you are really chasing the numbers and the performance I assume the impact will be heavy, and maybe it is better if you look for a better place (either less used or dedicated to training). If you just want to put some more effort, go for it and take it as a good chance to also improve your reflexes.