But if you don't accept the first or second one, they probably won't go out for the third.

If you change it you may want to have an attorney look at your changes.I use zillow or apartments.com to accept applications, both offer services to run credit and do background checks.For screening you need to set your criteria before you start advertising.I recommend reading a book on landlording, my favorite is "Landlording on Autopilot".

They told me that they can accept a higher rent if I can provide active leases (redacted info of the current tenants) that show a higher rent as long as it's a duplex. 

We require acceptable printed documentation of income (preferably showing year to date total earnings; hiring letter if new job plus last paystub), savings (they can redact the actual account number, but it must have enough identifying info to prove whose account it is), or whatever applies to their situation (passports, green cards, trust documents).

So you either have to work on lowering your operating costs or accepting lower returns. 

Whichever bidder is willing to accept the least amount of interest is awarded the tax lien certificate. 

I will say though when you accept an offer initially remember you still have gateways to pass through (Inspection, Mortgage appraisal) that have the opportunity to cost you.

You may be able to transfer your purchase option to another person, but that would need to be spelled out in your purchase option agreement, and you likely need the current owner's agreement to accept the sub-lessor as the transferee.You need legal advice here which I am not qualified to give.

I work with many investor real estate agents and know of good boutique offices that would accept and allow you to hang your license with little or no experience.

The first person gave me a 200$ deposit via e-transfer after I told her we accepted her application.