Duplex cables use one of the fibers for send and one for receive like I described earlier. To use each fiber separately you need more expensive transceivers that use different wavelengths to send traffic both ways over a single fiber. You don't need to do that here though.
Regarding stacking the Brocade ICX switches, I would defer to the post by @sic0048 above.
No, 2.5GBASE-T and 5GBASE-T are newer standards than 10GBASE-T so they generally don't work with these older network devices. You can however find transceivers that negotiate 10GbE with the switch and then those slower speeds with the device on the other end of the cable, doing speed limiting internally in the transceiver. Many of the chips don't handle that properly though based on tests here on the forum, so you want a transceiver using one of the Aquantia chips. Thankfully, the 10Gtek ASF-10G2-T (note the "2" in the model name) that you can find on Amazon or AliExpress is using the Marvell/Aquantia AQR113C and it generally works fine.
Another option is to buy a cheap unmanaged 4 * 2.5GBASE-T + 2 * 10G SFP+ switch off of AliExpress and basically use it as a media converter. The HiSource one can sometimes be had for as low as roughly $25-30, or as low as $18 if you are a new user. I own one of those and it works perfectly fine with both DACs and fiber transceivers. Using a switch just to connect to a different switch might be a bit weird though.
