Yes perfectly legal and safe to have one chain go from one side to the other, holding the axle in the middle ...
If your talking about using one REALLY long chain for the whole tractor, that "could" be possible, IF you used two binders, as in chain RF of trailer across the axle, add binder to tie into the LF of trailer, then have the excess lay on the floor of the trailer, loop the chain over the rear axle and hook the end of the chain to the RR of the trailer, then come back with a binder to tighten the chain to the LR of the trailer ... Two chains are easier ... I bought a 20' chain with hooks on both ends, and two extra hooks, cut the chain in half, added hooks ...
I prefer 4 chains, and 4 binders for my tractor ...
Yes, you should also put a strap/chain over the loader bucket, and the Backhoe (BH).
You can use straps if you prefer ... Just be aware of the working load limit (WLL) for example if you have a 5000 Lb tractor, you would need at the very minimum of 2, 2500 WLL rated chains/straps if you went completely from one side to the other with it tying into the tractor in the middle ... However if you secure it like I did mine above, you would need 4 chains/straps with a minimum WLL of 1250 Lbs ... Plus the 2 to put one over the loader, and another over the BH, or whatever rear attachment you have ...