It's been my experience since 2001 that any dealer who sells/installs Dinan software and parts will work with Dinan via the parallel warranty provided by Dinan. The dealers themselves don't provide any warranty, but they will not try to pass off problems to the presence of Dinan mods unless it appears that they may somehow be responsible. Most dealers will recognize that an intake or exhaust can't be blamed for a suspension problem, and so forth. Your best bet is to do some investigation of other dealers you might encounter and whether or not they are also Dinan dealers or mod-friendly in general. You are correct in saying that not all dealers are such.
2013 F30 335i M-Sport 8AT/MPE/MPPK, 2016 228i M-Sport Cabrio 8AT