I'm not really familiar how multicam strip works, but it's a bit weird that there are no buttons other than the "Source Channel" one until the strip is moved to channel 3 or higher. There should be some hint to why nothing is shown when the strip is on channel 1 or 2.
Also note that the old code shows the cut_multicam button for channel 1 if the strip is on channel 2.

I don't think PEP8 has strict rules here, but usually we don't add parentheses around single line if-statements.

For this specific case, I suggest to reformat this to:
if (i % BT_ROW) == 1:

Would remove the (strip.channel > BT_ROW) check here to avoid jumps in layout.

The empty operator-buttons feel a bit weird, we usually don't have something like that in Blender. I'd just change this to row.label() to add placeholder elements.
(You can also remove the sub setup above then.)

Note that this will solve the issue mentioned in the description (which only appeared with Region Overlap enabled). The operator-buttons invoked the sequencer.cut_multicam operator which allowed the event to pass through if no index was specified, just like here. When Region Overlap is enabled, we allow handlers from the main region to listen to events from the properties/toolshelf regions (if they haven't been caught by panel, button or operator handling) because these can be fully transparent. That's why the mouse-click invokes a operation in the main region, even if executed in properties region.