I enjoyed the whole of series three as well. At the moment, I'm taking the view that it's better for it to go out on a high and leave the audience wanting more than to run the risk of lowering the quality. Also, an awful lot of the people involved are very in demand with other projects - Toby Jones seems to be in every third film, play or tv show that I hear about these days for instance so it may also be that it would be difficult to get them all together for more filming.
I think that the coins being found will lead to the solar farm being at least delayed - the Roman mosaic floor being dug up like that was illegal (which was why Andy's evil boss deleted the photos of it off his phone, so he'd have no proof if he tried to alert the authorities). However, Andy invited the County Finds Officer to attend the rally on the field so presumably he or she is in the crowd of people going off to the pub at the end and can be called back and immediately informed.
About the way that the coins simply fell from the tree at the end: we were introduced to the idea at the end of episode one that magpies have some kind of supernatural power and observe human beings in the lyrics of "Magpie" sung by The Unthanks. You could argue that the magpies, as Andy suggested in an earlier episode, had spent centuries collecting the coins. When they thought that Andy and Lance were just another set of thieves after gold, they took away the coin that Lance found. But then, Lance and Andy made up with the Dirt Sharks, got the magpies' tree preserved and invited all the other detectorists to look for the treasure too. That convinced the magpies that Lance and Andy are "worthy" and so they let them have the coins in the end. Lance's discussion about Karma with his daughter comes in here, too.