First, your ISP will simply drop any packets with ULA addresses (fc00::/7, which includes packets your fd00::/64 network), so you cannot ping to your ISP from your ULA network.
Next, you are misusing the ULA addressing. You can locally assign addresses in the fd00::/8 range, but the next 40 bits after the fd must be randomly chosen to give you a /48 prefix ...
Packets are layer-3 datagrams, so the layer-2 frame encapsulating the layer-3 packet would need to have the host layer-2 address to get into the host network stack, where the packet would be discarded as not matching the layer-3 address configured on the host.
I am planning to use Cat.6 RJ45 pass through connectors
There is no such thing inside the Cat.6 specification. You should check out ISO/IEC 11801 or ANSI/TIA-568.
in an office network with around 70 LAN cables.
You need to use solid-core plenum cable for office deployment. Those are terminated in patch panels in a wiring cabinet and in wall jacks in the ...
Literally? No. Unless the devices were very close, it's very unlikely they would ever have been directly connected. Even 20-30 years ago, in the era of T1's, there were repeaters, digital cross-connects, and multiplexing into larger T-carriers.
As Ron has said, today the T1 is a relic. It will be emulated and carried as packets like every thing else "...
No. There may be a dedicated circuit between the routers and the nearest telco central office, but between offices they are switched and multiplexed onto other, higher capacity circuits.
Today, most T1 circuits are emulated over a packet-switched (IP) network. They are rapidly becoming obsolete, and are being replaced by SIP over the IP network.