can we declare local variables inside a!foreach()

Yes

Yes, [mention:0591d80bf3a5406ebeb3855605fe90c0:e9ed411860ed4f2ba0265705b8793d05], we can do that,
#For your reference, a small example.

a!forEach(
items: { 1, 2, 3, 4 },
expression: a!localVariables(
local!index: fv!item,
a!forEach(
items: { "A", "B", "C" },
expression: concat(fv!item, local!index)
)
)
)