Looks like you have a an extended gum between your upper front teeth because of which you have gap. So kindly get that removed first and a retainer should help.

Yes retainer will help you Consult with ur orthodontist

The gap seems to get close by retainer but it all depends on the possibility of level of attachment of frenum. Basically a frenum is a fibre of muscle underneath upperlip which gets attached on the gums just above the teeth in the centre. So this fibre poses a problem that even if the gap is closed by the retainer or any other orthodontic aid the gap will reapper due to the fibre. So first see that off, when you are having a consultation with dentist.

As the crowns are separate therefore a retainer can be used to fill the gap.

You are advised  to consult your dentist for the treatment protocol and select  the treatment options suited for you.

Yes retainers can be used.. Consult your orthodontist