Confirmation on query optimization and performance understanding (seek and scan)
To answer your questions:


Since you say 'all the records meet the criteria' I would advise doing tests on a larger set of rows where some rows are outside the criteria.
There are some circumstances where using an expression against an indexed field will result in the index being seeked, such as WHERE [MyField] LIKE 'SomeString%'.  However, using an expression directly in the field will result in the query not being sargable, and will induce an index scan.  For instance, WHERE LEFT([MyField], 1) = 'T'.
Not having a clustered index on a table (meaning the table is a heap) does not affect the ability for a particular SQL query to use a seek operation on an index.  Not having a clustered index on a table has its own set of issues that is way beyond the scope of this answer.  Check this blog post for more info on heaps vs clustered indexes.