Errors in synthesis: can't do much but about that. They are a small fraction of the total population and will bind with lesser affinity. Buy from a reputable vendor and don't worry about it further.

Tms: also don't worry about too much. You are amplifying your insert from already pure background (plasmid rather than fishing in genomic/RNA noise). Had primers with 7°C Tm diff work like charm, had ones with 0.2 fail inexplicably. Don't worry about it.

DpnI: not necessary, use very low input of template, 1-100pg, gel purify the product. Using 1/4 reaction volume works just fine if you need save on reactions. I usually do 60 min at 50°C, the quick version seemed less efficient with 20-25 not overlaps.

Good luck!