You probably don't need all the power you are trying to put into the shot. You just need a consistent, smooth, full stroke. Also, the spin you use is worthless without friction. Think about it. If there were no friction, it wouldn't matter how much spin you put on the ball. If you put too much momentum into the ball trying to get more spin, it will want to stay straighter instead of letting the spin do it's job because of friction with the OB and the cloth and the rails. A slower, smoother, straighter stroke will allow you to get more spin on the balls without putting too much momentum. It will also lead to more consistency in your mechanics, allowing for better speed control.
Simply practice shots where your stroke is consistent ALL the way through your follow through, and use a complete follow through. If you get in this habit, the rest of your game will tend to correct itself. You can never improve if your approach isn't consistent.
That said, what you do before your shot and after your shot matter not 1 bit in this regard. It's true. Having fundamentally perfect mechanics and routine, staying dead still until the ball is pocketed, etc. will improve your form. However, it is a long arduous road for most players to correct the habits they've formed from the very first time they've held a cue. You can walk this road if you want, but just keep in mind during the process that your #1 stroke priority is to keep your approach consistent throughout the time where the tip is in contact with the object ball.
That said, pay a little more attention to the patterns pros select ant the speed they use for each shot. Rarely do they put themselves in the positions you describe. Granted, these situations are not always avoidable, but I bet you that you can improve your game at least 50% without even touching your stroke. Not saying you should neglect it at all, as a consistent stroke is generally what separates pro from amateur, but keep in mind that you need to always be doing encouraging things with your game in order to reap the benefits of your success.