Would they sell more shares, and if so, at what price, and what would that do to the value of existing shares?
Yes, they will likely sell more shares. Why? Because like you pointed out, they would probably have to pay a hefty interest rate since they are not exactly financially sound. Also, their stock has been on fire. The offering will likely be priced close to whatever the stock is trading at when they do the offering. So, if we are in the low 40's, that's roughly how much they could do a secondary at.
It will dilute the value of shares that you own.
I'd hate to see my shares devalued by the issue of more. I don't even know if they can legally do that. But if they issued bonds at an acceptable rate, I'd buy some.
They can legally do that, and in fact they have already done it once (last June I think?).
Nobody "likes" seeing their shares diluted by a secondary, but you have to think bigger. If the company borrowed money instead and took on more debt, that would also devalue your shares (investors would pay less because there are few profits since some is being used to service the debt). If they did a private placement, your shares would be devalued (exactly the same as a secondary except not just anyone can get in).
Remember, if your shares are diluted a bit, but because Tesla has a bunch more cash they can bring BlueStar to market before anyone else enters that market and makes a fortune, then your diluted shares shoot way up, and you win. If on the other hand they have to wait an extra year to do BlueStar because they didn't do a secondary for fear of diluting shareholders, then another company beats them to it and BlueStar loses first mover advantage, then what? Your shares aren't diluted, but they also aren't worth as much either.
Give me the diluted shares that are worth more. The bank doesn't ask me if my money came from selling diluted or non-diluted shares when I deposit it.
Besides, if you are worried about your investment being diluted, you can always participate in the offering. That's what Elon did:
Tesla Shows How To Run A Secondary Offering, With Insider Buying (TSLA) - 24/7 Wall St.