That would be the fourth delay. But is it legally possible to delay Brexit again?
It is legally feasible to delay Brexit again, provided that all EU member states (including the United Kingdom) agree in the same way that was done for the previous three delays.
Whether it is politically feasible is difficult to say with any certainty at this stage, as a large part of this will come down to the UK election result, as well as any elections in other EU member states that may take place between now and then.
Legally, yes it's possible there is another extension, the same as before. The revised deal that Johnson agreed with the EU but technically has yet fully pass UK's parliament is in the same legal boat as Theresa May's deal, even though Johnson managed it one step further by getting the House of Commons to pass a 2nd reading of the draft legislation on his deal. But that Parliament is no longer sitting now.
So politically, it's probably too early to tell until after the UK general election. Unless you want an/the answer to delve into predicting the result of that election, and by implication of Brexit... Wikipedia seems to keep track of the latter. The November seat-level polling/models seem to point to a sizeable Conservative majority, perhaps enough to pass Johnson's deal thereafter. Interestingly, while the Conservative aren't doing much better nationally in opinion polls than in 2017, Labour is doing much worse for now, which seem to account for the seat difference.
And an interesting comment on Conservative remainers' electoral choice(s):
this group is also more likely than other party/referendum combinations to be undecided, with one in five saying that they had not yet decided how they would vote [...]
the Conservatives are banking on to ensure they hold on to seats such as Putney, south-west London, where the estimated remain vote was more than 70%, is that these voters will be too concerned about a Corbyn-led government, even a minority one, to “risk” a vote for the Liberal Democrats. [...]
The more the media narrative focuses on how unpredictable the election is, the less likely these Conservative remain voters are to feel they can safely switch their vote, in the end making the election rather more predictable than it first appears, and forestalling the Brexit realignment.
For what's worth it, regarding Brexit, Johnson has said that all future Tory MPs have pledged to back his deal:
Boris Johnson has said that all Tory parliamentary candidates have committed to vote for his Brexit deal if he wins a majority at the general election.
In an interview with the Sunday Telegraph, the prime minister said this pledge means that voters can be “100%” certain that a Conservative government will “unblock” parliament and deliver Brexit. [...]
Johnson told the newspaper: “All 635 Conservative candidates standing at this election – every single one of them – has pledged to me that if elected they will vote in parliament to pass my Brexit deal so we can end the uncertainty and finally leave the EU.
“I am offering a pact with the people: if you vote Conservative you can be 100% sure a majority Conservative government will unblock parliament and get Brexit done.”
As far as I can tell, all the MPs whom Johnson removed the whip from are not running as Conservatives candidates. In fact most are not running at all in this election, only one (Anne Milton) seems to be running as an independent. Another (Sam Gyimah) has switched to Lib Dems and is running in a pro-remain constituency. So at least the (known) intra-Tory opposition to the Johnson deal appears to have been effectively eliminated from the ranks of the Conservative candidates.
The only ‘binding’ document we have as of now is the United Kingdom’s notification of intent to withdraw, then-prime minister Theresa May’s letter from March 2017 which started the two-year countdown. Each extension was agreed on by both the United Kingdom (more or less) and the EU member states (unanimously). There is no provision in any of these that would prevent any further extension from being agreed upon; and if there were, the two sides (UK and EU) could agree to change that provision as it is all their agreement.
Therefore, the scenario I have seen perpetrated in jokes of a UK PM in 2119 travelling to Brussels in March as every year to present a letter to the EU asking for extension (‘nobody remembers where this custom originated or why it is being upheld’) is entirely possible.
The reverse is also possible. Assuming a long extension has been granted (say June 2020) but EU and UK manage to agree on a withdrawal agreement that comes into force on 1st March, an earlier departure than the negotiation period is possible (the wording of article 50 suggests that the two years are a maximum length, not a fixed period).
Furthermore, not even the document I gave the attribute binding is actually binding. The European Court of Justice has ruled that this letter was a unilateral declaration of intent of a sovereign nation and that further this sovereign nation is at the liberty of withdrawing their unilateral declaration of intent, stopping the Brexit process altogether.
Unlike ordinary mortals, states are not bound by any laws whatsoever (except for the laws of physics) as long as there is mutual agreement between them on what should happen. If the EU and the UK want Brexit to last for another 100 years, it will last for another 100 years. If both sides want the UK out tomorrow, it will be done tomorrow. Nothing in the current laws really matters, only the actual decisions of the heads of the 28 member states.
So the real question you should be asking is - is there enough political will to extend Brexit one more time? As of now this is unknown, as it hinges on the results of the upcoming UK elections. If Conservatives and the Brexit party gain a majority, the UK will be out very soon. If no majority could be formed, Brexit is likely to be delayed again. If Lib Dems win, Brexit could be cancelled altogether. Whatever the outcome it will depend strictly upon the collective will of 28 sovereign states, not upon some piece of paper someone signed a long time ago.
The delay could be expanded again until all the EU members and UK agree to extend it. Sometimes temporarry is almost definitive, like the Eiffel tower, that was built as a temporary structure.
Is interest for EU to have UK inside EU? Definitively yes, especially if the other option is a no-deal brexit. Uk is really interested to have a no-deal brexit?
I suppose that when all the parties involved will get a swiss-style deal maybe things will change.
About the border controls: Italy and Frange are both in EU and Shengen area, but last time I've made a trip in France on a bus, cops with dogs asked for an ID and took dogs to find drugs, so even in EU controls at the borders are possible.