The front-clip harnesses are just confusing to me... I, too, would wonder if the unbalanced 'twist' is a good thing for growing joints and muscles...
The back harnesses are probably worse - I put one on Denna the other day, and it was like having brakes with ZERO steering ability. I can only imagine the front-harness is like having a car that pulls to one side when you step on the brakes... which is not normally considered 'safe' operation.
On training...
I cringe when I hear mastiffs called a "large, stubborn breed"... they are NOT stubborn. They are Independent Thinkers!
As independent thinkers, they can take different motivators for training. Find your dog's top motivating object, and your training will go soooooo much easier!
If your dog is food motivated, that's easy! (thus, labs and other food-hounds are considered "easy to train").
High prey-drive dogs can be motivated with games of tug or rewarded with a few tosses of a tennis ball.
Mastiffs are guardian dogs. They are naturally independent thinkers (from being left to tend the herd or flock without human supervision), and are more often family-motivated, with high "pack-drive" tendencies. That means they respond to PRAISE and LOVE... but still tend to do something once - just because they love you, and then will return to their "job" as supervisor again.
I can motivate Denna with food (as long as it's GOOD food)... otherwise, I need to keep her in 'pack-drive' mentality, which means lots of love, praise and 'fluffing' to keep her engaged with what I'm asking her to do. If her mind starts to wander, I can voice my disappointment, and that will also return her focus back to me. If I have to resort to 'angry mom' voice, she responds VERY quick to my command...
For pulling on the leash - yes, a quick STOP as soon as you feel pressure will quickly let Lettie know that pulling will get her NOTHING. A Stop and then start walking in the opposite direction (with no words and minimal sounds to alert her to your actions) will also help return her focus back to you and off what she was pulling ahead to get to... and also will teach her that she needs to keep tabs on you at all times, or you might pull a 180 on her!
If Denna pulls too much, I stop and ask for a "Come Around" - which means she has to return to me, go around behind me from the right side to the left and stop in a heel position - THEN we will resume our walk. If I give the "ok" or "let's go" - she's free to surge ahead again (on her flexi)... or I give her a "with me" command for her to stick close to my side. She's smart enough to know the difference between the tension on a flexi lead and the pulling when she hits the end of it, too... she's also smart enough to know where 15' from me is - i.e. she RARELY hits the end of the flexi, and stops to sniff a bush at about 14.5' to give me time to catch up.
Anther thing I've taught my dogs when walking, is that they are NEVER to step off a curb without stopping first... When we come up on the corner to a street, we stop (without stepping off the curb), look both ways, then continue with a "Cross" command (or 'heel' or 'with me' or 'let's go' depending on situation). When you consistently stop at corners/curbs, every time, the dog learns very quick to anticipate the stop.
Mastiffs are really quick learners, not hard to teach, not stubborn... SMART! Smart enough that they get bored doing the same commands over and over again...