By that time (Lord Delaware arrived just in time to save them) they knew Virginia was pretty rugged so they needed to find out quick how to live there.
What they seemed to be unaware of was that they had landed just at the tail end of a 17 year drought period, so salt water was all the way up to the Fall Line. They did discover the Fall Line though, probably in their search for fresh water.
The folks in Maryland could simply have been folks dropped off by pirates, or maybe they were the pirates' support team, or maybe lots of Croatians. There are about 27 early European settlements older than Jamestown on the East Coast that have not yet been archaelogically surveyed.
I don't know how many of them are in Maryland ~ could be some.
Regarding settlement sizes, the local Indians in Fairfax County had a town of about 20,000 population near Beacon Hill (in Fairfax county VA). They manufactured ceramic dishes and pots in traditional Indian designs and styles until shortly after Jamestown was founded. As that area became available as a market for their wares, the Indians began manufacturing ceramics in European designs and styles.
That area eventually became part of Mount Vernon plantation.
You are undoubtedly correct. Consider that Columbus, Raleigh & most of the other explorers were working under what amounted to Royal contracts. Others, like fisherman, pirates, or lumberman, had probably visited before. Some would have stayed, willingly (to form a base camp) or by happenstance (shipwreck). These 'private' enterprises would probably have been subject to seizure had the monarch found out about it.