Should Betfair have a FIRST 5 market up on the National?

no

why not, there is a clamour on these forums calling for bookmakers to do so!

Race should be banned, would not bother me if it didnt have a win market.

if you want to back first 5 then find a bookie that does that - imo, waters down the market. I'm a gambler rather draw more for a place 1st 4 than 1st 5. Betfair does it its own way, do you want n/r's in 8 to 7 horse races paying 1st 2 like the bookies ?

Yes I think it's a reasonable idea. Certainly more interesting than Winners Card Number and some of the other ersatz markets. I am sure it would attract some reasonable liquidity, once the bots found it.
The only problem is that it could attract arbers.

FIRST 5, in addition to FIRST 4!

1st nine last year I'd have still got **** all !