1, 3, 8, 15) Agreed.
4, 6) This is a standard D&D 5e rule.
Well, even if its D&D 5e rule, is it really a good game mechanic with spells overlapping and preventing you from using them? I find both very limiting.
That's quite intentional. The reason you 'won' D&D 3.x by writing 'cleric' or 'wizard' on your character sheet was all the buff spells that did not require concentration or ways to remove the concentration requirement.
5e specifically wants you to pick one, and only one, spell to concentrate on at a time. That way the designers don't have to worry about exploit combos.
