2 questions:

1) how to add multiple filters. for example, show all records whose 'City'='Davis' and whose 'Zip'='95616'. if it can't be done in the user interface, hopefully you could give me some insight as to where to look in the xsl or the UDT source.

2) any way for the default grid view to offer a detail view on each record, as is the case when you switch to "AdvancedTable.xsl"?

1)a

in Grid View: enter a filter in Mange UDT 
City='Davis' and Zip='95616' 
If Zip is an Integer DataType, you would write:
City='Davis' and Zip=95616

1)b
in XSL

change
  <xsl:variable name="currentData" select="udt:Data[contains(*[name()=$searchColumns][1],$search)....
to 
 <xsl:variable name="currentData" select="udt:Data[udt:City='Davis' and udt:Zip='95616' and contains(*[name()=$searchColumns][1],$search)....

(In other scripts look for xsl:for-each or similar...)

2) This is not build in. 
(It is possible though not trivial and need some work. You can reference the same UDT module on another page and setup a customized XSL script as detail view. You can write an Calacluated Colum in your UDT that links from the page with the default grid view to the one with the detail view.)