Question about Value RJ45 T-adapter

No.

Good question!
Basically, you have four pairs of wires, of which only two are used up to and at 100 Mbps (and that is why the "trick" with this split adapter works). For >=1Gbps all wire pairs are needed. I.e. the prerequisite is certainly a dLAN with a net bandwidth of more than 100 Mbps so that all wire pairs are "handled". Then it probably depends on how the dLAN Bridge processes the data. Whether each wire pair is processed individually or together as a large data stream.
I would not assume that it simply works. It will probably come down to a trial and error...

Yes, that may all be so in theory. But the practice looks a bit different. I can't compare a dLAN with a full-fledged Ethernet cabling. So if the dLAN is supposed to deliver 500 Mbps... in practice, the highest throughput is 50 to 60 Mbps. This means that the question does not really arise in this sense. The loss of speed will not be noticeable, not even with real Ethernet cabling. (unless there are two servers etc.).
If you have such concerns, then take a switch for CHF 20.-- at Digitec.
Netgear GS605

Why should this not work? If the dLAN has an RJ45 connection, this can also be "split". I.e. "one makes two".