I'll let you read my post first as some of your recent questions may be answered there. Whatever isn't answered, feel free to ask again.
As for whether I think it's worth it for a seat if there are 4 adults flying as well...short answer, YES. We had 2 adults and a teenager with us and ONE toddler. Having a place where she was contained was so beneficial. She was safe from turbulence, safe from the flight attendants tray cart whacking her in the head/leg/arm because she was never sitting in the aisle seat flailing her body parts, kept from the glares of the passengers behind if she would have been standing on the person she was sitting with and looking over the back of the seat (this is what another toddler was doing part of the flight), she could nap super easy and without anyone having to hold her. The people a few rows up had a baby that fell asleep on dad sitting in the aisle and mom had to get something from the overhead bin which she couldn't reach so they spent a bunch of time trying to let mom get out to get to the bin, when that didn't work, dad passed the baby over and baby woke up screaming.
Not to mention that you'll have one in a seat and one free to roam about? Seems like it could open a can of worms with one upset they are confined while the other is bouncing, smiling and squirming all over as the rest of you are getting stressed out that the child won't sit still for take off. Of course, it could all go super smooth, just not something that can be guaranteed with kids! DD sat nicely in her seat for takeoff, and I held juice, water, gum and a couple other snacks to continually hand her so her ears wouldn't get sore.
ETA: not to mention that having your child in their own seat and restrained is a huge safety benefit if there should ever be a runway crash or failed takeoff, etc. Your arms are definitely not going to keep that child safe in that speed and force.