event disable HTTP rewrite

May be I am overlooking something but on first glance, both your iRules look the same. If you are trying to make sure that the client sees "a.domain.com" but the server sees "domain.com", you need something like this:

when HTTP_REQUEST {
if { [HTTP::host] contains "a.domain.com" } {
HTTP::host [string map {a.domain.com domain.com} [HTTP::host]] 
pool POOL-DEV2
}
}

when HTTP_RESPONSE {
if { [HTTP::header values Location] contains "domain.com" } {
HTTP::header replace Location [string map {domain.com a.domain.com} [HTTP::header value Location]]
}
}

You can potentially include the HTTP::uri logic within the iRule to achieve your goal.