In regards to land crossings, if you are headed to Kinshasa and Brazzaville the road to Matadi on the Angola side is shite and the most common route is to cross at Luvo (east of Matadi) as the road headed west from the coast (from N'zeto to M'banza Congo) is very good. The Luvo border crossing is a piece of cake. The road from Luanda to N'zeto is a bit rough in spots but not too bad.
Can't speak for diesel specifically but north of Luanda, there was a big above ground tank in N'Zeto that advertised 24 hour diesel, whether they had any can't say for sure. There is a fuel station in Mbanza Congo, whether or not they have fuel on the day you arrive is sometimes an issue. Road side sellers in the villages are fairly common, prices and quality is variable.
Having said that, isn't there a ferry that crosses the river at Soyu? I saw some large trucks that said they were heading toward Cabinda and they simply drove straight north when I peeled west so if you have your double entry visa I was under the general impression that there was a ferry that took you across and allowed you to follow the coast. Did meet one biker who came down that way but unfortunately didn't pay too much attention about ferrys and the such as I didn't have a double entry. Drive safe.