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Just saw your message. You don't include your itinerary info, so I assume this data will reach you too late. But in case this can be helpful to someone else in future...
I was there in May-June of 2009, so bear that in mind WRT to timeliness of details.
1) Never heard of any way to visit at night--i.e., after about 5 or 6 p.m. daily. Perhaps there are exceptions for special days, like special pageants or anniversaries or equinoxes, but your visit would have to coincide with these infrequent events. At any rate, I have no hard info.
2) If you're quite intrepid, there's an arduous foot trailleading from the town of Aguascalientes to the MP complex, which means you're not at the mercy of the taxis or scheduled shuttle buses that convey visitors up the switchback road to the ruins. (I met a French guy who did both the ascent and descent on the foot trail, PLUS the HP climb, during his one-day visit. He saw a guy take a bad fall on the HP climb.) Otherwise, the earliest buses leave from Aguascalientes (AC) around 4:30 or 5 am daily. You should buy your MP admission ticket in AC the evening before your visit; bus ticket you buy shortly before boarding at the bus stop.
3) If recollection serves, I had to apply at a hut inside the park by around mid-morning for a free permit to scale Huayna Picchu; one didn't have to arrive at MP at the crack of dawn in order to do HP, but the earliest possible arrival time at the gate is advisable if you want to have time to climb HP + tour and photograph/film the entire complex in one day. Myself, I would seriously consider paying the additional hotel bill and entry fees and giving the site two consecutive days if you're really, really keen. HP climb is well worth the effort--the MP complex spread out far below will look incredibly small when viewed through the clouds surrounding the summit of HP.
4. They're serious about not allowing tripods or monopods past the gate. (You can recover such "confiscated" items upon your departure in the late afternoon.)
5. A magical, mysterious place. I long to return...