Take the u bend off and check that's clear, is the sink draining OK? Can't see how a drain smell could be associated with a washing machine in that set-up.
No need for them to attend providing they have all of the necessary information, adequate measurements, wall construction, which walls are load bearing, first floor joists directions etc.
Looks like gable end straps and should not be removed. They should be fixed to the gable end studs and the horizontal boards. If they're not then you should ensure they are.
Watch the video from 1.34:
Probably permitted development, provided it meets the necessary criteria: https://www.planningportal.co.uk/permission/common-projects/extensions/planning-permission
Further more technical guidance here...
Can't really go wrong with something like the Malm range from IKEA IMO, they're chipboard, we still use one we originally bought in 2000. We have four of them now and two of those were bought secondhand.