Is there a way we could co-sign on a mortgage, or simply have the title to the home in his name and I transfer a potential down payment directly to him?

See following for more on this. https://www.biggerpockets.com/blogs/3441/51027-nonrecourse-loan-debt-for-self-directed-solo-401k-investment

If the deposit had not been paid and accepted, and the paperwork had not been 100% completed and signed, I don't see how this was improper.

I know I'm not giving you clear direction there, but thinks it's smart to really be specific on each property based on what it is, where, type of tenants, condition of the place, etc. 

Property taxes are a direct hit on your bottom line.• Low Insurance cost - landlord insurance in Austin is close to $2,000/Yr for a typical 2,000SqFt single family home ($350,000+) vs. about $450/Yr. for a typical 2,000 single family home (<$250,000).

I already have a portion of my paycheck sent directly to a savings account but that of course earns almost nothing with interest.What are some other interest earning options I can do for automatically saving money so I can buy my first deal in the next 12 months?

Sounds reasonable to me IF he is actually going to mentor you and be available and spend time pointing you in the right direction.

So please contact your cpa or tax preparer with direct questions. 

It may or may not show up in your credit report, but the payments and deposit will show up on your bank statements, also lying about it is fraud.You likely can't use delayed financing with it either for the same reason, it's gonna show up on your bank statements

I don't know if they will be able to open the line of credit through your property though, for that, maybe try looking up southern WI portfolio lenders, I'm not sure if they could do something like that, but that's what I would try, if anything else, try asking for a sales manager, they should be able to point you in the right direction, they will be pretty use to working with investors and may have the answer to your question.