Im typically pretty well diversified with real estate and stocks, but I think Ill be pushing my portfolio towards more real estate due to expected inflation. 

I am 17 and have been growing my investor mindset for around a year now, starting with investing in stocks and recently crypto and have been building my portfolio.

They can see the rise of inflation in the future, know they can't keep their money in the bank where it will lose purchasing power, and are trying to get that money working in assets like real estate and/or the stock market. 

Assuming a 1031 is not used (they may not exist in 15 years), do certain structures of owning the stock (Roth ira) make the stock investment more desirable from a tax perspective?

You are young, if you want to get wealthy in the stock market overtime then a financial advisor would encourage you to invest in growth stocks (appreciation) vs when you are getting ready to retire, switching out to something that has very little growth but pays monthly dividends (cashflow)It is very similar in real estate, appreciation will make you most of your wealth overtime.

My grandmother has a sizable IRA and is becoming more and more apprehensive about her traditional investments (stocks, bond, etc.), and more and more interested in moving her funds to a SDIRA to invest in real estate.

An index fund is a type of fund, like a mutual fund, that invests in a basket of stocks tracking a certain index.

For example, without this extension, an investor who realized a $250K gain from the sale of stock on April 1, 2020, would have until September 28, 2020, (180 days) to place their investment in an Opportunity Zone Fund.

I'm trying to get her to pick stocks. 

If anything slows them from raising rates or raising them very fast is the stock market crash they will cause by too much/too fast.