Thanks for posting the vid and photos.
I realise that this was your first try, but as requested, comments and suggestions.
If the mill needs up to 6 hands to guide and push-pull it there is definitely something not right with your setup.
If the mill and log are set up right the mill/saw should almost cut under its own weight and there should be no need for more than one person pushing. I'd start by placing the log on a slope - see my vid below
It also sounded like the saw was laboring more than it needed to on that size log. It would be interesting to see your chain cutter and raker profiles.
I reckon the fewer people around the saw the safer it is for everyone. Seeing as you appear young enough you should try to learn to operate the mill on your own.
It would be much easier to operate your mill if it had a few more handles added to it then you won't have to bend over and push on the rear mill rail to drive it. Have a look at some of my mill designs for handle ideas.
If your dad insists on helping put a helper handle on the outboard end of the mill for him to hang onto (he should only need to apply light pressure on the helper handle with his left leg) but keep him away from the body of the mill. The operator needs to see the full length of the bar/chain when starting the cut and having somebody in the way blocks the operators view, Having arms and body anywhere blocking and operators view is a risk not worth taking.
The cross bars on the stihl mill look too flimsy for the mill to be used with an 084 - this photo shows that the cross bars are bent and they also look like they are bending further when starting the cut.
I guess is why your dad is helping? My guess is they also bend when the cut is finishing - this will lead to a crooked start and finish. If the cross bars bend even slightly when starting this means the bar will be under some stress when the back mill rail tries to perch up onto the log top and this will damage the bar and chain. I think this is a major design flaw with that mill - I would remove those cross bars and weld or bolt on a couple of much stronger cross bars in their place.
Removing those thin cross bars means you could then use log rails for every cut. Then you don't need any help to start the cut, just perch the saw on the rails, start it and then slide it into the log.
This was my first go with my BIL mill
BIL_MILL_maidenV.mov - YouTube
This extreme slope is fun but not necessary, even a few degrees of slope helps.
I'd also suggest everyone near the saw uses a face shield or at least eye protection, if something drops onto the chain and it flicks the object into face or eyes they will know about it. The first time I used my BIL mill a bolt came off the mill and landed on the chain and flicked into my face - lucky I had a full face sheild on.
I'd also suggest muffs over ear plugs, or preferably both. There has been some pretty clear research that suggests that ear plugs that are equally rated to ear muffs are in practice only at best 75% as effective as muffs. Even professional audiologists cannot consistently fit plugs to their supposed full effectiveness.
If your dad insists on sitting in this position while the milling is taking place I'd strongly recommend he wears full wrap chaps.
When starting out and fresh it is probably OK but after while concentration drops off and one slip and his legs are in big trouble.
If a helper handle is provided on the saw nose it will get him away from that dangerous poistion