Just saw this offer in OLCI... along with 85€ for an upgrade, seems to only be available on 'regular' economy rows behind the emergency exits.
For ~1hr flight I'll skip, but interesting offer nonetheless.

Sounds like KLM is now also actively pushing this option. It was reported in August that AF was doing the same:
[NEW?] Blocked Seat Option at OLCI

The reason why this is only available for seats further back is likely because they will be trying to get 2 pax to pay for this benefit, and put them next to each other. Meaning that for this to work they need a row of 3 to be available, which is much more likely further down the back of the aircraft. Effectively you are not paying for an empty middle seat but only half of an empty middle seat.

I was offered the same checking into economy comfort, thought it was great until I was asked to pay, could make that a bit more transparent on the first screen.

I'm not sure how that would be possible, I assume that once paid then the middle seat will show as blocked in the seat map and the next pax who would like to use this option will have to go to the next row.
There are plenty of empty rows available in the purple zone between EC and emergency exit which would have made it a more tempting offer over a free EC seat. (with a currently empty middle seat, which they could also offer this on...)

Got this also at the weekend

It's shown in your first post.

You're being offered 24F (for free), or 24E&F (for a fee). You're not being offered 24DEF, so someone is already sitting in 24D.

Either they picked 24D as normal (for free), or they may already have been offered the 3 options; 24D or 24F (for free); 24DE or 24EF (for a fee) or 24DEF (for a greater fee).

I'm not saying this is conclusive proof that they have done it this way - but the system is set up to allow this. You don't know for sure whether the person who selected 24D bought a "free seat beside them" option or not, but it's entirely feasible for them to offer the same deal to a person wanting to sit in 24F.

Now that they are proactively offering such offers, presumably the seat map offering is tweaked to allow at least 1 or 2 rows to be "held back" to allow such offers to be made.

But I could have just as easily picked 24E.
Of course they could then refund the person in 24D if they paid for an empty middle seat but that seems a bit stupid.

Mostly I'm wondering what happens if I pay for a whole row and once in the air someone in a middle seat decides to move to one of the seats I paid to be empty. Do I get to kick them off? Will the crew support my claim to the seat? Do I get refunded if not?

My advice there would be - sit in the aisle seat for as long into the flight as it means nobody would bother moving...

They wouldn't offer you the option of choosing 24E if the person in 24D had paid for a middle seat. It's not clear from your screen grab that you were actually able to select 24E in that specific case.

Regardless, it would be a very easy system tweak for them, in the case where 24D bought an empty seat 24E but 24F is still available, to also offer the person considering 24F the option of buying an extra seat. KLM already knows it will remain empty (because 24D paid for that), but that doesn't preclude them from inviting 24F to also pay to have 24E empty.

That's exactly my point, and I agree I didn't add a screenshot of the seat map, let's see if they have the same offer on the return.

I think it would be a bit odd, how would 24E looks like in that case? they will need to find a way to not allow someone to select it directly yet it needs to show as empty so the person booking 24F will be enticed to pay the extra amount.

"Not available"; there are almost always 1 or 2 rows at the back shown this way no matter how early you book a flight

(The below is the seatmap shown during the purchase of a KL flight scheduled for 15JAN25)

Ahh, interesting... that looks exactly the same as an occupied seat though unless you actually hover it.
That would still not entice me to pay for it though

Or the middle one (assuming you paid for the row) Otherwise someone may ask to squeeze past you to sit at the window

Offering this benefit free of charge to ultimates would make me fly more on afkl. Offering it free of charge to platinums would make me very happy too. In our company we just don't believe in upsells and for a commuter it can add up from your own pocket.