Thanks for your response and further questions
It will be the latter gain that is considered (so the gain made just from April 2015 to sale) as the gain that arises from you leaving the UK until 05/04/2015 will be preserved under the old residency rules.
So yes the variations are real and correct as one looks at the situation had you been UK resident throughout and the other looks at the changes of legislation and how that affects you - which is merely the gain made from 06/04/2015 to estimated date and value of sale - so your gain is set to be £1500 estimated
There are no capital gains exemptions or reliefs beyond the annual exemption allowance of £11,100 (so you will have NIL capital gains to pay based on your estimated position)
Yes if the property is jointly held then each of you share the gain arising and each of you have the £11,100 exemption (so with a gain of estimated £1500 - thats £750 each - less the annual exemption allowance (due for each of you) so NIL capital gains to pay
No you cannot use personal allowances against a capital gain just the annual exemption allowance as indicated above
To get an accurate value pre April 2015 - two local estate agents or the district valuer - link here for them (but HMRC would accept 2 x local estate agents post period) https://www.gov.uk/government/organisations/district-valuer-services-dvs
Finally through out affiliation with Just Answer, we cannot take on clients met through this service directly - but if you are seeking just information and guidance to mange matters yourself - then do come back to Just Answer and you can always ask for me