Run tamoxifen at 20/20/20/20 and clomiphene at 50/50/50/50, no need to frontload it for a week as full peak plasma levels will be reached for both within a few days. If you are blasting the HCG after cycle and do not run it on cycle I would increase treatment to 6 weeks for both.
12-13 weeks is fine, 10 is fine, I was simply recommending 15 weeks so you could get more out of it, it's not necessary by any means, do what you would prefer to.
Yes you need to taper off arimidex due to estrogen rebound, the drug does not eliminate E2, it only inhibits it for the duration of it's active life which is fairly short (reason for the EoD frequency of use). It's best to gradually lower the dose for the few weeks leading into PCT as your testosterone levels drop. Dropping the dose down by half in the second week after last pin and then changing to an E4D approach would be the best option. Some may need an AI during PCT, some may not, a blood test can determine this.
If you are asking if there is any negatives to running HCG, clomid and nolva together yes there is as HCG is supressive, it is a lot more optimal to run it on cycle. If you are asking if there is any negatives to the drugs themselves of course there is, but the benefits of using them far outweigh the negatives, and the negatives are fairly uncommon except for elevation of E2 with HCG and clomid which almost all people will get.
If your money is on long term health it's best to not cycle at all realistically, medical professionals advise against AAS use for a reason. I don't care what any online guru spouts; AAS are counterproductive from a health standpoint. Are they overly life threatening? No, but that does not mean that there are no implicating personal risks involved. It's a risk vs reward type scenario, you need to be the judge of whether steroids are really for you as one cycle almost always turns into two and so on.