There are plenty of mortgage brokers out there that will shop your loan to multiple investors/lenders to get you the best deal and the seasoning will not be an issue.Talk to @Andrew Postell - he is very active and can help you with the back-end loan.Andy

If you have debt, try to pay more than the minimum payment - even a small amount can have an impact.Don't close any credit card accounts - part of your score involves how much credit is available to you.   

This way there is no legal issues involved.

Generally, if your middle FICO score is at least 700 the conventional loans make more sense, and sellers prefer to work with conventional financing whenever possible so this would help you in a multiple offer situation.FHA and conventional financing do have options to roll in the rehab costs to your financing they have significant downsides to them, but it would be worth it for you to research these programs if you have not already done so. 

The  benefits of the SOLO401K are the much higher contribution limits, and the fact that you can be your own 'administrator' (not sure if that is the correct term) and that IF you want to borrow non-recourse money there is next to no paperwork, or tax, involved compared to the SDIRA. 

I've flipped a few houses and I'm familiar with the work involved and costs, etc...

That usually gives me enough room to win some, lose some, and still net out around equal.Also, as you get involved in this silly little game you'll start to get a feel for what it costs to do certain tasks and you will naturally be able to get a feel for costs. 

What policy we used depends on the extent of your involvement. 

For people with multiple properties, what's the experience like during a recession.

much less work for multiple doors.