Alright, lots of questions. If you are new to lifting, I suggest you get a reliable training partner to learn with, or better yet get someone who knows the exercises and safety, etc.
The real issue with HIT for a beginner, IMO, is that you are still learning form. Using bad form without a spotter and training to failure is not a good idea, even if training to failure is considered safe, which I do consider it safe.
For me, when I have a spotter I know I push a bit harder. I keep pushing even when I think I can't get another rep. Often you will get caught half way into a rep and be able to push through it, sometimes it will come crashing down on you. Plan for the worst and be safe. If your form suffers too badly or you start jerking/swinging - you are done. Safety is more important than anything - an injury takes a fraction of a second and will derail progress for weeks or worse.
If I come to the final rep I can do, and I push it up and complete the rep, I always try to hold that rep up as long as possible and fight the negative on the last rep.
If you have a spotter you can complete forced reps on SOME of your workouts, and if you don't have a spotter drop sets are a good option on SOME of your workouts. You don't need to, and shouldn't, use these every single workout on every single exercise. They are taxing.