In plumbing a PVC drain system, I'm wondering how much strain can be put on the assembly in order to get proper slopes, connect fittings, etc.

Here's an over-simplified example just to illustrate the point:
Say you have a toilet at one end of a bathroom, and a stack 8 feet away at the other end.
You have a 90deg elbow out of the toilet and towards the stack, an 8-foot straight piece of pipe, and then another 90deg elbow from the pipe and down into the top of the stack (again, oversimplified for illustration).

If you solvent-weld the elbow on top of the stack directed towards the toilet (based on reference marks), but you are off say a deg or two so that a straight pipe out of that elbow will be 2 or 3 inches to the left or right of the toilet elbow, is it acceptable to torque the whole assembly in order to make the connection, or will that force, over time, cause a leak? If this is really NOT acceptable, how would you go about actually ensuring that the angles are precisely correct when glueing? (Another example might be forcing a slope in a pipe that, when glued, maybe had less than the slope required).

Thank you so much for any help.

Putting a continuous strain on any joint is not good practice.
If you have a 8' run to the stack you will set the center of the fitting on the stack at 2" below the center line of the elbo under the toilet. Normal slope is 1/4" to the foot, Good luck, Tom