They provide a nice centralized marketplace for syndications, which could otherwise be tougher to find.Now that I know this exists, and due to the fact I’m accredited (so I can invest in all kinds of fun stuff more directly than ownership of a public equity) I am wondering if there are other reputable platforms that facilitate direct investment in other alternative asset classes.

You’re right, and I have a good reputation around my area.

Consequently new construction is almost exclusivley limited to 450 to 650k homes.

In addition to choosing one with experience with 203k loans I would vet lenders and look at their local reputation.

For example, if you sell a property for $1mm and receive $900k in cash and carry a second mortgage for the final $100k how does this affect the 1031 and tax consequences?

Is it a greedy seller who wants top dollar for a pile of poo, or someone who is actually going to work with the guys/gals who are going to execute the project and have their butts/reputations/credit scores on the line?  

Justin, You can rollover your 401k into self-directed IRA, but not into HSA, which is Health Savings Account and not a retirement plan.There are several reputable providers here on BiggerPockets who are active on the forum, just search the forum for related keywords and you will find out who they are.

SFHs, not being purpose built rentals, can easily be sold without any negative consequences.

Elderly owner of 20 unit complex selling portfolio but suffered significant tax consequences ( assume capital gains) when sold her previous property. 

The team seems reputable, but it's also my first investment property and I don't want to get burned.