Short-term swings in currencies and stocks can be huge after French elections

How low can the EUR/USD could go if Marine Le Pen wins the French election? 

The first round this week is unlikely to have a huge impact, unless the polls are completely unattended and Le Pen having an outstanding result with her share of votes go over 30-32%. The EUR in that case can be heavily impacted in the short term and fall as much as reaching 1.0520.

And how high can it rise if Le Pen is not the next French president?

The impact to the upside would be less noticeable that in the opposite situation. In a tight race after the first round, I would not see any evident transforming before the second round.

How big of a risk will the French election be for the markets? 

As mentioned, the risk is there and especially in the short term the swings on the currency and stock markets can be huge. Also the bond yields in the US and Eu will be impacted, with the spread between the bonds of the strongest economies and that of the peripheral EU countries widening sensibly

Which is the best tool to track French-election market risk? French bonds? Polls? EUR futures?

As mentioned the spread with the Bund in Europe.

Could the French election have a GBP-Brexit-type effect on the EUR?

Again, for this round, it will depend on Le Pen having the lead and on the size of that lead.

Should traders prepare for important gaps in the Euro on the post-French-election Monday opening?

Gaps, thin liquidity and widened spreads are certainly possible. Traders should carefully assess their exposure and reduce the size of their trades if necessary

Is the European Union "two-speeds" idea good for the EUR in the long-term?

No it isn’t. Europe can only move forward united, and only by immediately implement all the action needed to render the Euro a currency working for all members, no matter what Germany thinks.