We want to keep everything legal and above board, so we're steering clear of any properties that aren't zoned or grandfathered for the intended use.

I have already checked with the city, and there isn't a process for checking the readings online or anything, so my only option would be to drop by the property and look at the meter myself.

Any feedback on online classes vs. in person?

need to look online ,search REIA groups or go on meet up.com ,home owners association ,HOA association   and if there is no group , start one 

Has anyone seen success from online advertising, or do  physical yellow letters increase the perceived legitimacy of one's investing operation to a prospective buyer?

Yes, there are legal ways to do this.

You will learn about legal processes / land descriptions.

I do have a legal background.

They are doing the same relatively minimal amount of work regardless of how much capital the LLC holds.A good self-directed plan facilitator or attorney will have one or more preferred custodians they work with, as well as tighter integration between the legal and custodial processes.

You can legally tell them that you are not interested in setting up a showing for them.