I don't use fb either; closed out my account around '09 or '10 and haven't regretted it one bit. Although I can see why businesses need it to build an online presence. So if you want Seedman's take on your questions, you'll need to find another method to contact him. Maybe email him or use a friend's fb account.
Without watching you dip under a challenging load (because anyone can display textbook form with light weights), I can only make educated guesses.
It could be that you're: 1) engaging the stretch reflex in the connective tissue; 2) by shifting the load to the connective tissue, your muscles are briefly disengaging AND resting. Both these factors, when combined can make any movement feel easier. After all, the body will always look for the most efficient path. Bodybuilding is a different animal in that we're often trying to do just the opposite.
The forearms should stay relatively vertical.
However, there's no need to keep the elbows tucked directly to the sides of the torso as Seedman states. 45 degrees OR less is perfectly acceptable and you can still engage your lats.
You might want to look into using dipping stations have the bars flaring slightly out. Face inward, so you're looking at the part where the bars taper in. This setup will accommodate a slight to moderate flare - the exact angle you can only find through experience - without putting undue stress on the wrists and elbows.
When the elbows are tucked directly to the sides, most people WILL have to shorten the range in order to prevent excessive migration of the humeral head. Remember, we want to minimize this for long-term shoulder health. Therefore, the angle I mentioned can buy you a little more depth - IF you have the shoulder construction, overall mobility, and discipline to do so - and keep you safe.
Another benefit to this approach is you'll most likely engage your pecs better. The main concentric action for pec major is horizontal adduction of the humerus, the upper arm bones. By leaning forward and moderately angling out the elbows as you descend, you're better placing the pecs in a stretched position which means they're more likely to engage - as long as the stretch position isn't extreme or held too long as this can down regulate them. As you press, the humerus are moving towards the midline; this is essentially horizontal adduction.
Just don't go extreme with this method by using the Gironda dip or variants. I'm convinced there are very few individuals who can do this type long term and stay healthy.
As I stated in my first post here, a few can go slightly below 90. As you go deeper, the risks increase but not the reward.
Try it with bar setup and moderate elbow flare I mentioned earlier. You might be surprised that 90-100 is all you need for a viable compromise between what's effective and what's safe.
Yes, use pain - the wrong kind - as an indicator that you're screwing up. But I take things to a more sophisticated and proactive level. And you should too.
Let me explain.
There are meatheads who ignored the early warning signs and drowned them out with nsaids, fish oil, and rationalization - because real men don't take time off - until they inevitably pay the price and get to wear the broke dick hat.
And there are people who felt perfectly good that fateful day. The warm ups and initial working sets go without a hitch. Then, seemingly out of nowhere, that hideous pop and they realize they need to change their zip code to Snap City. We can do a post-mortem and figure what why they injured themselves; it's usually a programming error. Regardless, these cases are more difficult to predict. This is what keeps me up at night researching how to best stay healthy.
Obviously, in our quest to become stronger and more muscular, we have to take risks. But they must be calculated and any unnecessary actions must be recognized and discarded.
Always ask yourself: how can I maximize the gains and minimize the danger? Am I ego lifting, is my technique shit, do I even need this movement for my particular goals, am I working out imbalances, taking proper deloads? These are the questions you should consider.