Well the very fact that you can tell me that you've been doing a certain kind of thing for quite some time means that somehow you've misunderstood the central concepts. Nothing should really be planned out in advance. Only the parameters that are able to be used are planned but what you choose to do with those are decided on the ground.
Just adding weight and never adding sets or reps is not SDT at all. You have to add TO THE BASE VOLUME. If you drop the volume to add weight, then what happens is you are doing basically a short linear periodization. When you add weight, you add it to an existing volume, reps and sets.
In your above scenario, you took away a set, so that you did 4 reps with the 150 instead of 8. So even though you "added" a set of 160 X 2, you did not add to the base volume so there was no SDT progression.
Your second scenario is a grey area. This is hard to explain but the simplest way to say this is that with SDT we always assume that the time given to the base volume is kept constant. In other words, you are not adding rest and therefore more time. When you take a set of 4 and turn it into two sets of 2, with rest in between, you are still doing the same volume, yes, but taking more time to do it in and thus decreasing the density. It is hard to argue that decreasing the density here and there makes a big difference in the big picture, but if we allow that to happen all the time, the whole system becomes pointless and there is not real way to measure progress. Look at it this way, it is easy to add weight if you add rest periods! So that is not the point of SDT.
For the second scenario, if you had done this kind of thing, it would have made more sense:
150lbs X 2 X 4 reps
150lbs X 4 reps
160lbs X 4 reps
Single progression - weight added
All that being said, I've noticed a trend with most people defaulting to these fairly low base volumes of 2 X 4 or 2 X 5. That is actually a fairly low volume from which to progress well. SDT is better suited to more moderate volumes of around 15 to 24 reps, but not exclusive to that. Reps should be 4 to 8, but with higher reps being possible depending on the movement. Isolations for sure can use higher reps.