If they used a loan for a primary residence, there may be some ability to enforce something but that really doesn't impact your decision. d) There is no effect on capital gains since they didn't use it as their primary residence for 2 years.

I've already put down a significant deposit and now I'm considering leveraging a 100% loan for a second property, with a budget of up to $600k.

Though you are looking at a spec home so already built and I'd be surprised if the builder is offering you a 30 year amortized loan at 5.99%. 

In some places, you'd take this directly to a real estate agent or a title company, in some states attorneys handle contracts.

Unless you can do some damage with about 30k which is how much you can potentially get based on your estimated value and remaining loan amount.

Roseville have a everything schools, hospitals, restaurants, high tech companies like Bosch, Penumbra, HP, Amazon Hub, Mckasson, Oracle (Rocklin) with three zip codes  95747, 95678 & 95661 and Rental is HOT here.

I am currently seeking equity partners and/or personal loans as viable options. 

Is it feasible to obtain a loan or a good old boy agreement where a lender offers 100% financing for both the purchase price and construction costs at hard money rates, secured by a second position on the borrower's other properties?

Quote from @Heather Kiddoo: I'm happy to jump on a zoom and talk about what TC's do in the creative real estate transaction if either of you are interested @Mike Hern @Chris Seveney Thanks for the offer, but we have an in house team and use attorneys / escrow companies for all of our real estate transactions. 

You need to talk to a Banker or Loan officer to go over your credit and see what you have in savings.