*I was there last year, second trip, fantastic place. Some answers to your questions
Solo is good but shared is better, especially if break down or have an accident.
Even if a road starts out sealed there are constant roadworks, landslides and a general lack of maintenance, you will encounter dirt. But some roads are new and in great condition.
Learn some basic phrases, I found a guy from Vietnam living locally who was more than happy to help me with some language lessons plus he wrote some phrases in English and Vietnamese in a pad which were very helpful.
The road from Sapa to Lai Chau is good and take you over Vietnams highest pass theTran thom pass
I think you will get accommodation most places, depends on what standard
just stay clear around the time for Tet celebrations, lots of Vietnamese tourist both local and from o/s
And be careful of speed limits especially in towns around 40kph, one of our group got busted
People are very friendly as long as you show respect, enjoy your trip.