I think, you need the combination of formulas. The answer is:

={QUERY({A:C},"select Col1, sum(Col2) where Col1 <>'' group by Col1"),{"filtered sum";ArrayFormula(IFERROR(VLOOKUP(UNIQUE(FILTER(A2:A,A2:A<>"")),QUERY({A:C},"select Col1, sum(Col2) where Col3 ='yes' group by Col1") ,2,0),0))}}

Explanation

It's not hard if you'll take it by parts:

• ={basic query, {"header"; vlookup(a, help query, 2, 0) }}

Basic query

QUERY({A:C},"select Col1, sum(Col2) where Col1 <>'' group by Col1")

It's simple, I've used Col1, Col2... notation to make it work with any range.

Vlookup

IFERROR(VLOOKUP(UNIQUE(FILTER(A2:A,A2:A<>"")), help query ,2,0),0))

We count sums with criteria (c = 'yes') in the help query.

UNIQUE(FILTER(A2:A,A2:A<>"")) part of the formula gives you a list from column 'a'.

Help query

QUERY({A:C},"select Col1, sum(Col2) where Col3 ='yes' group by Col1")

Here you may enter any conditions what you want. In this case it's Col3 ='yes'

Your second version is correct. For example,

=query(B:G, "select B where " & H2 & " = 'Available'")

works correctly in my spreadsheet when H2 contains a letter such as C or D or E.