Tom,
1.'What were Emmet's top half dozen designs?'
To be honest, I do not know as much as I would like to about his lost courses; however, I will give his top 10 (considering the present GCGC to be a Travis course with the routing and several greensites Emmet):
1.) St. George's GCC
2.) Leatherstocking GC
3.) Wee Burn CC
4.) Mohawk GC
5.) McGregor Links CC (original course)
6.) Huntington Crescent Club - West Course
7.) Meadowbrook Hunt Club
8.) Congressional CC
9.) The Seawane Harbor Club (original)
10.) Pelham CC
The last couple could be interchangeable, Salisbury #4 (Eisenhower), Pomonk appears to have had some wild greens, Northport had some great holes along the marsh, The powleton Club had/has some wild greens, and to be fair, I have not played Huntington or Congressional. Also to be fair, I do not know enough about some of his high profile lost designs Queens Valley, Hillcrest, Fenimore, Cooper River, Lawrence Village had some great holes along the water, Old Westbury has some wild bunkering (DW Missing links has a layout off of a scorecard from The Golf House), and I would love to know more about two of what would appear to be his more interesting designs that appear to have little evidence of them left: his estate course Sherrewogue and his Cuban course CC of Santiago (I have seen a picture from an add in The American Golfer).
2. 'When did Tull join Emmet...and was he a positive or negative influence IYO?'
I believe Tull started working with Emmet around 1924 and his odd bunker style (free flowing amoeba-like) started creeping into their designs around 1925. Positive influence, I am not sure, I like Emmet's bunker style a lot more than Tull and you can see the disparity at Cape Cod CC. I know that Tull helped move Emmet toward lengthier courses like Huntington Crescent Club. I am not a fan of Tull's greens a lot larger than Emmet nor his bunker shapes or positioning. I would have liked to have a little bit more length and the Emmet style (may have helped some of them being lost or built over). Tull also renovated several original Emmet courses and eliminated alot of that quirk i.e. Pelham, Cape Cod, and I am not sure, but I know he renovated Port Jefferson CC extensively.
3. 'I know Emmet spent a lot of time in the British Isles and I know he was friends with Macdonald...do you think his design style (with a British/quirky touch) was a result more of one than the other (UK/I & Macdonald)? '
I know this site is big on hands-on-experience and I have never golfed accross the pond. Thus, I can not directly determine the influence. However, I know they do not mind blindness in their designs and Emmet used it a lot in his designs. I know in describeing the orginal briar hall main course (BH also have a 9h course) that he said one of the holes was inspired from the Gate Hole at North Berwick (this is from an article in the American Golfer.) I would guess his quirk comes from over there as well. I wold suspect that while researching famous hole in the British Isles for C.B (who was a member at GCGC and won their club championship one year if I am not mistaken - I know his name is on a plaque for winning something in their clubhouse) that he got a flavor for golf course design and his philosphies and that set him on his path.