Marriage certificate:
I was in a similar situation but not visa for Italy but for France. Note, french visas are processed by TLS contact and
NOT VFS
My marriage certificate was all in English. Requirements were the same as that you found for Italy - i.e. attestation required as documents issued from my country not inline with Hague convention. Note:
India follows Apostille convention, see
list of countries
Just to cut a long story short, if your embassy is not willing to attest on the marriage certificate, see if you can get a photocopy of your certificate attested from the embassy. Though these are two different things - getting original attested and getting a photocopy attested, it is better to go with something and see if VFS accepts it. This is just a suggestion as I am not sure about VFS.
The way to get it attested properly is to send it to Foreign affairs or similar institution in your country and get it attested from there on the original. You then take the certificate to your high commission in London and then they will happily stamp it. But this is so of course not feasible given you just got to know the requirements now ... Next time you go to Pakistan you could get this done.
Multiple entry:
I think it is unclear when they issue multiple entry. I got a 6 months multiple entry for my second Schengen. But these things will definitely boost your chances: 1) previous Schengen visas, 2) Annual multi-trip travel insurance, 3) your status in UK, 4) .. and ofcourse you asking for a multiple-entry visa and perhaps provide a cover letter explaining why?
Visiting other countries
With a single-entry visa, you are only allowed one (i.e. single) entry into the Schengen zone. Once inside Schengen zone, you can hop in-out of countries as long as as you stay
within the zone.
Tickets
Technically, booking tickets should be fine as they cant oblige you to buy the tickets before you travel. I remember reading somewhere on forum someone booking tickets and still getting a visa. But you have to have confirmation of a return ticket (preferably purchased) before you submit your application for visa.
Finally, it is outrageous, but remember to take 4 photocopies of your own passport before you go to the appointment. You will need two for your own application, one copy for your spouse's application, and one copy each for your children. And photocopies of all supporting docs. They look at the originals and send photocopy with application.
