Interesting chapter. I liked how you demonstrated Harry's reluctance to reveal himself; keep drilling that in, that he fears for Maria revealing herself more than his friends being offended. Maria's life is much more important than their satisfaction with him. On that note a scene where Fred and George pay back the Dursleys in a... permanent manner... as a gift from all of Harry's friends could be very funny and great plotwise. Nice job on fleshing out Maria, but what else aside from learning has she been doing while Harry has been away at school? Did they communicate daily during the year, or was this a summertime/owls only thing (and if you go with owls, defend on how his friends never came across a letter. I mean, Hermione is defintely suspicous enough to notice Harry writing all the time to someone, and even Ron would notice Harry recieving owls in his dorm room. Make it good if you go through with communications during the school year). Did she travel around at all while Harry was off, or just stay in England or even just hide away in Thorne manor?
I missed Dumbledore's humility that he had shown last chapter, which I would have thought to return this chapter. I was looking forward to him coming himself to fix 'his' mistake. Ah, well, you can't have everything. Tonks, as a half blood and auror makes sense to send after the dursley's, as does Moody as the one with the inital threat. But why the heck did you send Snape? Maccie G would make much more sense as an order rep as she is not only the Deputy Headmistress and Harry's house head looking out for her student, but is also a bit more intimidating. I mean seriously, a frigid, harsh looking schoolteacher who knows how to discipline unruly students would be much more sensible to send (as the Dursley's haven't been following Albus' rules) than a greasy, dark looking man... Unless Albus sent the reformed death eater to do some muggle torture, in which case it makes perfect sense to send him to extract information in the most painful manner possible, but in that case why send aurors along?
As a note, be careful how you portray Snape. Right now he is still close to canon based, but many people who try to truly redeem him fail badly. Just be very careful of how you portray him: Snape is a vindictive, narrow minded, slightly petty man. So far you have nailed it but watch out how much he empathsizes (feels for) Harry when he discover's the truth. I can much easier see him striking out at Harry for wasting Snape's time and not working at his potential than I can him applauding Harry for the slytherin attitude; regardless of how cunning Harry might be or how smart he might be, Snape does NOT like potters and stereotypical gryffindors, both groups of which Harry belongs to. He would take it as a personal slight that someone from these two hated groups got one over him, not proud. He also does not like to reclassify people (eg: he has belittled Nelville because of his initial impression and has refused to break his mold that Nelville fits in) and thus would not take him on as a protege or slytherin-in-griff-clothing. Snape is for snape's gain, not Albus', not the Dark Lord's, not Malfoy's, not the Order's, but his own gain. He only wants Potter to defeat the dark lord and disappear. On the other hand, he would love to have control over Harry because of the power it would grant him (legal power over Maria==emotional power over Harry if used right==power over masses bwahahaha, not to mention). Make sure you emphasize having him ploting over Maria.
Keep up the good work.