Yes. If your visa says 'No Work', it means just that. Working in the 'black economy' circumvents both the VAT and National Insurance schemes.
To begin, let's look at this comment from the UKvisas Independent Monitor...
"...Linked to the allegation of vagueness is an associated one to do with a lack of booked hotels and tickets: I am aware that some Posts advise all applicants bar transit passengers not to book tickets or hotels until they have their visa. I endorse this approach. It is not right to require bookings which will cost money before a visa is obtained and UKvisas has now issued advice which states explicitly that applicant should not be expected to make travel or hotel bookings before obtaining their visa..." --Report of the Independent Monitor 2004 (Immigration and Asylum Act 1999), published February 2005, UKvisas
So the answer is No. Round-trip air tickets are not required to apply for a settlement visa, or any other type of visa for that matter (the exception being air-side transit visas which are outside the scope of this FAQ). The Foreign Office tells us that purchasing tickets in advance of an application is not recommended because of complications that can occur.
Yes, but not with a visa issued to a different sponsor. You will need to reapply first.
Yes. But to come back in to the UK, you would need to get a spousal visa first.