ajohn

Olive-backed Pipit and Dusky Warbler are both very widespread and should be easy to see. Olive-backed can be found anywhere with both trees and open ground (even some urban parks). Dusky Warbler prefers low grass/scrub, especially near water.
The other species you mention are not always so easy to see. For White's Thrush try wooded areas (Tai Po Kau can be good, especially the picnic sites, although the birds may get disturbed by people). Citrine Wagtail is easiest at Long Valley - there are often 3 or 4 birds present if you take the time to check through all the Yellow Wagtails. Little Bunting prefers open country areas - Lam Tsuen and Long Valley are perhaps the easiest sites, but there are others.

Gulls in HK don't visit rubbish tips and fishing ports in the same way as other countries. Although you can see them offshore, your best chance of good views of gulls is at the boardwalk hides at Mai Po.

ajohn

As Richard mentions, the tide is all-important for shorebirds. Based on the predicted tides for next April, I think you would be better coming the week before (i.e. 14th-21st rather than 21st-28th). The tides are high in the middle of the day, which means that birds should be pushed off the mudflats onto the roost sites at Mai Po. Even if the tide is lower than predicted, birds should still be close to the hides. This week is also likely to be good timing for Spoon-billed Sandpipers and Nordmann's Greenshanks. Most shorebird species will be around and in breeding plumage, although some of the later migrants may not be in large numbers (and it may be too early for White-winged Tern, which is a late migrant).

Also there may be a better chance of wintering species if you come earlier (including a chance for those you missed before - Citrine Wagtail, Little Bunting and possibly White's Thrush)