To what extent is stat maxing possible? Is there such a thing?
I got my Lilithmon to +100 int and she's level 99 now, am I done training her int?
Yes, if her Int is (+100) and her ABI is 100 that means you're done, only int bonuses she can get now are from Attach equip items. If She's brainy (which gives a TINY amount of int to her as well if you really care about stat maxing) then you've done as much as you can for her.
In Summary for maxing stat, the 3 things that are customization:
Attach equip items
Bonus stats
Personality.
Personality affects base stats in a small way, bonus stats are what's in brackets and attach equipment adds on to that which is why you may get say (+110) if you have an equipment item that says "+10 int" and the bonus stats is already at the max of 100 bonus stats overall.
Gems are safe to sell, yes.
Digi Islands cap at 10 Digimon and 5 levels of boost to hold more items.
You get the items that boost the levels of islands as well as the item that adds new Digi Islands from Quest rewards.
Maaaaybe a chest too but I haven't seen one yet that I recall.
Yes, you get both farm expansions and plugins from chests. I just got several from the final dungeon.