First off - the scale of the issue: "It emerged last year that some £38,000 of spending had not been declared to the watchdog, which the Conservatives put down to an "administrative error". A £20,000 fine could be imposed."
And how many in question? Sky News on the 15th reported "Files from 12 police forces relating to expenses during the 2015 general election are passed to the Crown Prosecution Service."
According to the Independent, "an MP would be disqualified only if he or she were sentenced to jail", and furthermore continues "In the only recent case, Fiona Jones, Labour MP for Newark, was found guilty of fraud in 1999 for failing to declare her full election costs, but her conviction was overturned on appeal". In other words, no, May would not lose her majority unless the MPs in question were to head to jail.
Furthermore, "There is evidence that MPs of other parties might have overspent on their election campaigns too." - "The Scottish National Party’s helicopter, which visited many constituencies during the campaign, was charged as a national expense.". Predictably, nothing has happened in this regard.
Interestingly, so far the only party fined over 2015 election expenses is... Labour. BBC reported back in 2016 "The 8ft "Ed Stone", carved with ex-leader Ed Miliband's key pledges, was among £123,748 of payments missing from Labour's 2015 election return. A further 33 receipts, worth £34,392, were missing, the commission said.". The associated fine was £20,000.