as for the line above your red line, I've had that happen when I was recording key frames, i.e. AL thinks something has changed with the actor from the previous key frames and stuck a black line in, but I've never had it happen once I've clicked and recorded both key frames, when one appears that I don't want, I just do is copy and paste the key frame from where the gap should start into where the gap finishes, but like I say once I've recorded the key frames they stick correctly for me.
Grouping - that's personal preference, for me - if the groups are required from the start I group then one at a time into actors/groups and don't bother with AttachToGroup, if I was constructing something like a car and animating putting the wheels on before driving away, I'd use attach to group once the wheels were in place. but its one of those things where whatever works best for a given situation - there is no right or wrong way.
SofDev used to have more animations on their site many years ago, but then they re-jigged the site and they were not put back on, they haven't been available now for quite a few years. - what do you think you are missing ?
As for the Excavator, you can move any actor at any time you wish (within reason
), i.e. you can have the second arm or bucket to start moving before the first arm has finished its movement - I assume that's what you mean by overlapping, I overlap things all the time.
Hope this helps