I worked for an engineering firm before retiring. Your comments about some folks reluctance to try new software hit home with me. I dealt with our engineers and project-partners at remote offices and with subcontractors and clients. Trimble connect was never accepted as a method for managing a model’s development.
At a minimum, when dealing with reluctant 1st-time users, I would send a link to the them to download the viewer. If SU models are large and can not be emailed, then ask the engineer if they currently use a cloud server and ask to give you permission to a folder on it; if not then setup a folder in DropBox and then the send them a sharing link via email.
If they won’t even try the viewer, then the 3D warehouse could be answer, but often this did not seem to be acceptable; especially if your are working on multiple projects and need to provide restricted access for each project that involved proprietary designs.
If not the 3D warehouse, then you could only be left with sending them screen captures or exporting jpg,png, or pdf files for them to markup manually or digitally and (scan the markups if necessary) and sending them back to you. And, yes, some would often import the jpgs or pngs into PowerPoint and for do markups!!!
For more “flexible” customers/engineers, they would used the Viewer and markup screen captures. If you are careful and setup relevant scenes with proper titles, then you can walk them through issues via telephone or skype; eg. " … go to Scene15, Pump Room Cross Section". You can also add textual notes/questions/comments at particular scenes.
Better yet, if they are willing to use SU FREE and the label/text tool to add notes. This makes communication much simpler! Stress to them the economics of faster, clearer communication; especially if there are budget restrictions (you have quoted so many hours to complete the model).
You can chew up a lot of time adding specific notations and many detailed scenes or Layout; sometimes spending as much or more time than actually doing the modeling (this can be especially frustrating when design changes came into play).
If the design/project will cover a long period of time or if you have short deadines; hopefully, you can convince them to buy a subscription for SU since it would more than pay for itself and give you and them greater flexibility.
IN ANY CASE, whether using SU free or Pro and/or cloud services, creating a video (with an app such as SnagIt) demonstrating and explaining only the use of the most basic and relevant tools and toolbars or menus would be to your advantage. Often times, I found that it encouraged more interest in using SU to expedite a project.
If you go to the 3D warehouse and search for “jsc_associates” you can find a some engineering projects where Scenes are used to clarify a model or design elements; eg., see the “Solids Handling Building” or “San Jose WYE Junction Chamber”.