Looks to me like this was a bud graft - probable edges shown in red. That's fine, except that the second picture shows that the remains of the rootstock has died back and this could spread down. Normally a small rootstock stub like this stays green and healthy and is cut off as the top variety grows. If the plant was supplied with this dead piece, I would complain. I would now carefully snip off the dead piece without damaging anything that's green. So all the new growth is the Caracara variety. I think there's too much coming from this point and would remove some of the thinner new shoots. But it depends on what shape you eventually want - a lower bushier plant or taller and more tree-like.

You can do it either way; if you leave some of the mandarin and graft to that, the mandarin part then becomes what is known as an internode (you can google that) The problem with internodes is you now have three varieties, namely the rootstock, the internode, and finally your preferred variety; and the odds are that they will not all grow at the same rate, leaving you with a somewhat ungainly plant. Sometimes growers will intentionally graft in an internode, in cases where the desired scion is not compatible with the desired rootstock.

I actually have one of these in a large pot. It is attractive but obviously, wickedly barbed. I have seen mature trees growing in Zone 6b climates, so they are hardy, but not so common (guess it's the thorns!). The original Citrus tree that was grafted to my Poncirus died and I have just grown it along over the years. Curiously, mine, which gets protected some from the Winter is evergreen (I was surprised to read that the species is deciduous!)

Yes. Understanding that the longer the run the more line loss you will have is important though, and add to that the current load on the circuit and you will have 3 parts of the answer. There are charts that describe how long a wire can be to carry a load at a given temperature and if you need to size up the wire past a certain point to keep it or the contacts cool. Safe loading is 80% of what the line can load to, to prevent heat and fire. A 15 amp circuit then would load to no more than 15x.8 follow? Or since Volts x Amps =Wattage then 15a x 110v = 1650w and safety dictates 1650w x .8 = 1320watts. Deduct light and transformer wattages (all devices) and length of run deduction from there. I should say my driveway is quite long so it influences my answer but your situation is different I would guess.