Training to failure isn't bad per se, it's just that a lot of people can't handle doing it frequently. Going to failure on one or a few exercises every few weeks probably won't kill your workouts, so if it's something you enjoy, go for it.
One key thing I don't think we discussed yet is diet. If all weight gain has stopped, you should eat more. If you're eating adequate amounts of food you should be gaining something. If training is off it'd be more fat than muscle, but you'd still be gaining weight. So one thing you should definitely do if all weight gain has stopped is up your calorie intake. That's another reason I find people stall. They often think it's the workout, but they're not taking into account the added muscle they have and the added caloric demands that puts on the body, and also people don't often take into account the caloric demands the workout itself puts on the body. It's a lot of eating to get to a calorie surplus somethimes.
That's a very good point. I very well could have stopped making gains as a result of diet.
I started out 9 months ago at ~30% bodyfat, so my main goals in the short term have been to lose weight and acclimate myself to lifting. I'm now down to ~20% bodyfat, so I'm happy with the results so far.
I've been realistic up to this point in that I knew I wasn't going to make huge muscle gains while trying to lose weight. I also knew that because I was just starting out that I was going to make gains regardless of the specific method of training I used. But as you said, a calorie deficit isn't conducive to packing on muscle. I'm at a crossroads, so to speak, where I really want to increase my gains but still want to shed the excess bodyfat. One thing I've been doing is eating more calories on lifitng days while eating fewer on non-lifting days (usually within 500 calories of what my body requires).
Now that you've brought this up, it might not be a half bad idea to switch to doing this on a weekly basis where I eat more on heavy weeks and eat less on light weeks.