Just a couple of notes.
"So for the English Baptists that were present at Westminster (or at least petitioned the Assembly with respect to baptism)..."
There were no Baptists in the Assembly. They were not invited. There were, however Paedobaptistic Independents (i.e. Congregationalists).
See this link for a list of pastors and theologians:
http://www.apuritansmind.com/WCF/AssemblyMembers.htm
Baptist churches can still be considered true churches so long as they do not hold a heretical view of the Lord's Supper (and some baptist churches do), or obviously some cultic or heretical doctrine of fundamentals. Ecclesiology will not unchurch a true church. Biblical Ecclesiology prevents independent sectarians from being an ecclesiological train wreck, but that will never a church. Bad theology, or the wrong administration of the sacraments will.
On the "Presbyterian Reformed Baptist" thing, if you are considering this according to ecclesiology, that fish won't swim. It just doesn't exist. If you mean according to the sacraments, then simply say "I'm a baptist attending a presbyterian church." There are a number of independent churches that still hold to non-instrumental EP but you just have to find them.
On the "your church is in sin" thing, well, that is something one cannot get around if they are either a Presbyterian, or sectarian. One or the other presses the issues with the other team that they ahve made a serious mistake in either the sacraments (Christ's work in beleivers) or ecclesiology (Christ's authority over beleivers.) If one is right, by necessity, the other is sinning. But again, that does not unchurch a church unless it is a matter of cardinal truth - like an issue around salvation, grace, justification, law and gospel, the nature of the Supper, etc.
Just some thoughts.