You keep asking the same question, not new/additional ones. I would suggest you download the LoRaWAN specification and read it, especially the parts regarding the receive windows. (RX1 and RX2)
A1. Yes multiple devices can send join requests at the same time using different frequencies and spreading factors.
A2. One of the 3 join requests is going to fail because the gateway is only able to send a response to two nodes.
A3. RX1 and RX2 are NOT on the gateway. RX1 and RX2 are on the node and refer to the time window the node is listening for a response. The gateway ‘just’ sends packets at the predetermined time, it does not know if that packet is targeted at the nodes RX1 or RX2 window.
A4. See A3, the gateway does not know about RX1 and RX2. It just gets a packet from the back-end that it needs to send at a certain time. As there is only one transmitter only one packet can be transmitted at that time, so if multiple nodes are listening for a response at the same time window only one can get a downlink packet.
A5. Nearly correct. The gateway gets the data a short time before it needs to be transmitted, each packet contains a time stamp telling the gateway when to transmit it.
In a (final) attempt to explain the problem:
If node A transmits a join request at moment T it expects a response at T + 5 seconds -/+100 milliseconds (=RX1) or at T + 6 seconds -/+ 100 milliseconds (=RX2).
If node B transmits a join request at moment U it expects a response at U + 5 seconds -/+ 100ms or at U + 6 seconds -/+100ms)
If node C transmits a join request at moment V it expects a response at V + 5 seconds -/+ 100ms) or at V + 6 seconds -/+100ms)
Now if T equals U equals V (all nodes transmit at the same time) TTN can instruct the gateway to send a response at T + 5 seconds and at T + 6 seconds. There is no third option as none of the nodes will be listening for a response at a third time interval. This means only two nodes can receive a join accept message.