1. the powered ones do more "swipes" per minute, so in general they tend to be more effective. If you're good with a manual brush, you will most likely get the same results
2. enamel is far too strong to be abraded by the brush, what does get abraded is the dentin (the yellowish part closer to your gums, that's why sometimes you can see small dent-like abrasions there) so try to focus on brushing softly in those areas. A good idea is to use soft bristles
All you do is good and you're probably already doing 95% of what can be done, the other risk factors are smoking, female hormones, stress and diet. If you are not stressed, don't smoke, are male and eat mostly plant-based whole foods you can't do much more than that
This has to be weighed against anecdotal evidence ( good anecdotal evidence is IMO better than your average study/research) from vegans. Vegans tend to have very high occurrence of cavities despite the best of oral hygiene. ( source ex-vegans on their blogs etc.) Eskimos when first discovered by western explorers had a complete absence of cavities despite bad oral hygiene and a diet of almost no plants (Source: A book written by an explorer, whose name I cannot remember)
2. enamel is far too strong to be abraded by the brush, what does get abraded is the dentin (the yellowish part closer to your gums, that's why sometimes you can see small dent-like abrasions there) so try to focus on brushing softly in those areas. A good idea is to use soft bristles
All you do is good and you're probably already doing 95% of what can be done, the other risk factors are smoking, female hormones, stress and diet. If you are not stressed, don't smoke, are male and eat mostly plant-based whole foods you can't do much more than that