I worry a bit about Memrise as a tool for learning words.
Dumbledore went over the curb of the lawn and up to [I would have said 'kama'] the entrance door of the house (? all these apparently doubled 'lon's and 'tawa's are confusing. Why not just 'tawa (or 'kama lon') lupa tomo' Am I missing a point here?) He put Harry down in front of the door [Ahah! 'lupe tawa' for "door" to differentiate from 'lupa lukin' for "window". Sorry] and got a letter from his overcoat and put this letter inside Harry's high clothes ['seli'?] and went to the other two [why 'lon', again? -- or maybe why not 'kama' for 'tawa'?]. These three waited and watched the little pile of clothes for fifteen breaths [works out to about 15 seconds unless magicians are slow breathers]. The top of Hagrid's body fell and rose. Prof. McGonagall blinked rapidly. The light on Dumbledore's half-moon glasses was not bright ['suno ala lukin']
Finally, Dumbledore sais "Well! Our work is done. We have no cause/reason to stay here ['tan ala pi awen (pi lon) ni']. We can go to the party?" ['lon'?]
"True" said Hagrid, as though he was talking with a cloth in front of his mouth ['len pi lon sinpin'] ["talking through a muffler"?] "I need to get rid of the motorcycle. Good night, Prof. McGonagall, Headmaster Dumbledore." [vocatives first, apparently. and 'o' after the name, not before.]
Hagrid did not cry using the inside of his gloves [?doesn't quite compute] and mounted ['lon'?] the powerful bike and started the power of this vehicle using his feet [didn't he just start/turn on the bike?].
Dumbledore said "Prof, McGonagall, ['o' after noun] I think that I will see you in a little while [or "for a little while", I never get those straight]. He bent down and raised his head toward Prof. McGonagall. Prof, McGonagall breathed through her nose to her glasses. [? hard to describe indeed]
Still pretty smooth, but with a couple of places where I don't see quite what you were saying, so just went with what made sense to me in context.