Didn't you post about this a few days ago? It's exactly the same scenario as one within the last week, even down to the structural review, the woolly comments, and the former colleague being managed out. If so, there really isn't much anyone can add to the last comments - nobody can tell you how long a process it is. They must tell you what needs improving and how they will measure that improvement, and how long a review period there will be. They should have a procedure policy, so you need to ask for that because that should outline, as close as possible, what steps and how long each step may take.
A grievance won't help you at all - if it is their judgement that your capability is in question (and I am not taking sides on that matter, simply stating a fact) then they need to follow a capbility process, which it appears they are doing - you may be aggrieved, but you don't have anything to complain about in that.
You can ask what severence might be on the table - that is for you to decide. It may be nothing more than what you will get anyway, although a good reference is worth money in the bank.
Resignation won't change the PIP - they ought to continue with the process during your notice period.
A lawyer isn't likely to be able to do anything more than charge you right now - if you are thinking of legal action (which may be limited as an option) then save your powder until you have some sort of case.
Just be aware of the fact that if this went to a tribunal (and based on bvery limited information here ) provided they follow a fair (in law) process you are likely to lose. A tribunal is not allowed to substitute their own judgement for the employers in the matter of capability, so unless they are extremely perverse in setting targets or judging perfromance against them, you would have to show the actual process was unfair, which is a tough one.