When are you travelling?

1. Yes, the barriers close about 30 minutes before the Eurostar's departure. All you have to is to get in the Eurostar terminal by then. Then you're good and still have plenty of time for the secutainment and passport checks. They state 45 min to 1 h time, in order to spread passenger flow.

2. When getting of the Thalys, get down from the platform to the main concourse. Almost right next to it You'll see the Eurostar terminal. It's one or two minutes walking. You will be almost immediately there. You can't miss it, it's very flashy and very British (platforms 1,2 or 3).

3. Depends on the amount of passengers, it's usually done in 5 to 10 minutes. You'll spend more time in the waiting area anyway , than at the check in. 

If you're unsure about it you can try to rebook on an earlier train, or take the reservation free IC from Amsterdam to Brussels earlier. 

As long as the inbound train is on time you will have enough time, just.

Check-in is just waving your reservation/ticket at the scanner. It is the security and double immigration that takes the time.

That should be enough time. If you miss the Eurostar because of a delay, they're obliged to rebook you onto the next available train without any extra costs, because this is a transfer that should work.

The Eurostar check-in and secutainment is in the station.

Transfers in Brussels Midi are straightforward and only takes a few minutes between platforms. Since Thalys and Eurostar are the same company they will ensure your connection or find you a seat on the next available. Although they close checkin at 30 mins before departure they ensure that everybody arriving by then will clear passport control or again be found a seat on a later train.

The general feedback is Eurostar staff in Brussels are extremely helpful if trains are delayed.

Eurostar tends to use platforms 1 and 2 and thalys seem to arrive on 4 or 6 so should not be too far. 

The check-in is only really closed 15 minutes before departure.

In case it's useful for planning the brisk walk, station platforms and Eurostar location map can be downloaded at the bottom of this page: https://www.b-europe.com/EN/Stations/Brussels-Midi

Live information about arrival platform/track also available. 

Eurostar entrance looks like:

I have a similar issue at Brussels Midi. The question is whether 20 minutes is enough time to transfer from the Eurostar train to an IC train going from Brussels to Liege? The Liege train is the last train that night going to Liege. This is our first time ever at the Brussels Midi station.