Willoughby is friends with Sir John, so Im sure he often shoots on his manor. Sir J didn't say "Oh has he been poaching my birds.". he was pleased to hear that his friend was in hte country.

Possibly he had heard taht there were some new poeple in teh cottage and felt he'd like to meet them as Mrs Smith is an elderly boring lady, but I can't see waht's wrong with that.

As for "seducing girls without a male protector" sorry but I can't beleive this. If he were dong this sort of thing, he woudl be barred from society. Men did not seduce well born unmarried girls.
As for the "keeping tabs on Marianne".. people were expected to socialise with other gentry. Col B had IIRC a home in London and spent time there. He and Marianne might not meet Will that often but unless they lived all the time "in the country", it was likely that they were going to be at the same parties at times. It would look very odd if they were known to be enemies and pointedly avoidng each other. He and M would IMO spend time in London as S&S is one novel where the characters DO visit London as a matter of course as most gentry able to afford it did. unlike say P&P where the Bennets dont go there....
I dont see that it meant he was "keeping tabs" on her that he made her his standard of perfection or spoke admiringly of her as a beauty.