A cocktail dinner often involves no sit-down eating at all (often, but not always). Guests are served all or most of the food and drink by wandering waiters, and there may or may not be a bar or a buffet table. There is plenty of seating, and lots of small, low tables. The usual idea for this style of catering is to encourage people to keep getting up and moving around, having conversations, but not being nailed into one social group for most of the evening.
The last cocktail dinner I went to had lots of food served by wait-staff every 20 minutes, three dishes per "course." I don't think any guest had some of each&every dish served. As the evening went on, the "courses" were spaced out further, to every 30 minutes or so, then every 40 or 45 minutes, etc. My DH and I arrived later, so I don't know how many courses there were all up -- but it was at least 8 courses, plus carvery and dessert table. We were there for about five hours, and the pace of service at this dinner was consistent with others I've been to.
All alcoholic drinks were served in glasses, on trays carried by wait staff. There was never a time when there were no drinks being served. All soft drinks were available from an ice-bar, and included fruit juices, flavored sodas (pop, fizzy-drinks, etc), soda-water, bottled water, and ice. Not all dinners break down the drink service this way, though.
Foods served at this dinner were mostly chilled or room-temp finger-foods, some were hors-d'eouvres, some were more substantial -- two choices were small chinese-takeout-type boxes with different hot stir-fried stuff inside, served with chopsticks -- the third "dish" for that course was also in a take-out-style box, but was french fries with a sort of BBQ sauce as a "dip" or "gravy" on it. Another course had all pate-like foods, served on large fancy spoons -- one of the pates was served on porcelain Chinese-style soup spoons. There seemed to be a theme for each course, but I couldn't figure out what they all were.
There was a small carvery for a couple of hot roast meats, served buffet-style, set up after the first three "courses" of tray foods had finished up -- plates and forks/knives were set up at the carvery.
There was also, later in the evening, a table of desserts/confections set up near the carvery, all were cold, all were finger-foods -- tiny tarts, little cut squares of fancy cakes, lots of jellied and layered things with pretty garnishes, gelatin-glazed fruit cups, and chocolate-covered fruits. In your case, this sort of thing would probably be set up after the birthday cake had come out.
I hope all this detail helps you. Good luck with your party!
How lucky you are to have a beautiful place available that you already know, to use for your party!
When the time comes, tell your DH "Happy Birthday" from the folks on Y!A. ..... :-D
 I forgot to mention, there were as many staff people working at tidying up all through the party, as there were people serving. They may have been the same people (serving their food & drink assignments, then tidying up before the next course?), but I don't know, because it was a very large party. Just another bit of logistics to consider. Have fun!