If I follow your first question correctly, Yes your taxes are based on amount reported on Line 15 Form 1040. This amount would include income (and allowable losses, if any) from all sources as detailed on Schedule 1 and its various supporting forms and schedules. In your case, as an LP who did not actively/materially participate in the LLC/MMP rental business, your passive activity loss is limited to your passive activity income, in your case ZERO.
Now for the second question, it "feels" as if entering K-1 amounts is a waste of time. But not so. First its required. More importantly, your currently unallowed passive activity losses carry over year to year until you have passive activity income to offset the losses. Also, losses from one passive activity can be used to offset gains from another activity.
And finally for the third question... this question is why I previously suggested you might want to seek support from a tax professional. Hopefully you had cash or property distributions and you certainly benefited from that.
