Looks like the wider community is not too fond of the grind either. We are 3 days from the end of the event and not even half-way to getting to the 10 trillion points required for the maximum gold bonus.
I think the higher bonuses are not supposed to be reached. They are here "just in case", to keep a goal for players. The true last one is at 3.250 trilions and it was reached early compared to the last Nidhogg event were the last good reward was reached few hours before the end.

For mother (at least the last part of Scene.3), very defensive teams works well. I soloted it with RG Pink Miko. After a certain point, it reach it's cap damages and with enough defense it's just can't kill you with le life drain.

For Laeva, it's hard. Very hard. I just went with all my higest power and pray to kill him. As you say it's a damage race. You need to win the two first battles with taking the least damages possible (and keep your best Crit damages alive) and send all your forces on Laeva to deals as much damages as possible before the turn 3. There is no actual strategy. You just need power, power and more power.