The key here is that as soon as you can afford a real attorney the prosecution often won't go forward. If you take public counsel, poor people in the system call them 'public pretenders' because the vast majority of the time they are buddies with the DA and will just press you to take the plea deal. Ive read about a few exceptions where they actually represent the clients but the vast majority of the time they just pressure you to take the deal and threaten you with the full penalty under law if you don't deal.
This is wrong on several counts. They can't force you to talk at all. A subpoena can't compel you to talk. They can't compel you to talk because you have a lawyer. And they certainly can't compel you to take the stand.
If you mean that they won't torture you if you refuse to testify at trial, you're correct. But the judge may declare you in contempt, and throw you in jail. And if you continue refusing, there's no limit to jail time. It could be for life, if you're determined enough.
Absolutely not. You cannot be forced to testify against yourself and you cannot be held in contempt for refusing to do so.
If you are testifying against someone else there are instances in which you can be compelled. Even then you can't be compelled to testify about anything that would implicate you in a crime.
Actually, in most if not all US states, you do need to explicitly state that you're invoking 5th Amendment protection. And, as you can see from my GP comment, I already acknowledged 5th Amendment protection.
Once you invoke the 5th you cannot be compelled to talk or testify against yourself. Full stop. In your post above you said "they can subpoena you," which they can't. You said in that case "you do need to answer questions," which you don't. And you also said you'd have to answer questions "at trial, on the stand," which you don't.