Is there a way I can "co-sign" with one of my parent's credit cards and be on their credit history in order to boost my credit score? 

I have two banks that have done so as well as a credit card, and know of a couple of organizations that have as well.

•Use business credit or 0% interest credit cards strategically—just be careful with repayment timelines.Next StepsThe biggest thing you can do right now is surround yourself with experienced investors, agents, and lenders who have done what you’re trying to do.

So an investor buys the property, you pay them a down payment for the lease option then pay rent for a year and after a year you have option to buy it.What makes this challenging is the lease option payment would probably need to be around 20% of the purchase price but it sounds likeyou do not have it. maybe moving is just not in the cards.

My MIL owns a floral business and apparently the credit card she uses for her business is keeping her from refinancing.

Once I sell that I’m gonna convert to a fourplex and do the ADU.going through cash quickly and also using credit cards when possible like to purchase materials.

Often the best risk, hassle and tax-free approach is to pay down debt.If you have a car note, credit cards, etc pay those off first. 

It's likely a scam anytime they sound like AI or the grammar is weird or they use the word "kindly", if they ask for you to respond to them off-platform, if they say their employer will pay for it, if they ask if the dates they've specified are still open (when they can clearly see they are open on your calendar), if they ask for a video of your property (they probably want to set up a duplicate listing with your photos and video and scam others out of money), or if they want a long term booking and they have just joined the platform and have no reviews (could have been evicted from somewhere else, and hope a newbie will let them in with no background check, they'll end up not paying or credit card is bad).  

These cities literally helped build up this country and our economy and I will never understand how much negativity they receive for fighting through adversity and playing the cards they're dealt!

If you play your cards right, this could be a great way to build equity and add value to the property.