According to Chabad custom, must one be careful not to take three steps back if the person in the back of him is still in middle of Shemonah Esrei?

Yes, as is the ruling in Shulchan Aruch, which is binding on all Jewry without exception, and thus the question itself is most puzzling. In fact, the Alter Rebbe was so particular in this matter that he writes that one who doesn’t follow it will be banished from having audiences with him. Sources : See Admur 102:5; Shut Admur 12 Takanos Tefila and Takanos Liazna

How important is it to give an exact report to the insurance company, and may one exaggerate expenses, or not report negligence, in order so the insurance covers more costs?

It is biblically forbidden to do so due to the stealing prohibition, whether the owner of the company is a Jew or Gentile. One must give an exact report to the company, and may only receive reimbursement for money that he is entitled to under the contract, based on the true events that occurred. Source : See Admur C.M. Hilchos Gezeila Ugineiva Halacha 1-4

Buying a new computer keyboard for Pesach

Question: [Sunday, 8^th Nissan, 5781]

I noticed that my computer keyboard contains a lot of Chametz in it, as like most people I eat lunch and snack while working? My question is if I can use it during Pesach or if I should simply buy a new one or try to clean it?

Answer:

One is to purchase a new computer keyboard for Pesach and designate it for annual Pesach use. If this is not possible, then one should shake out whatever crumbs he is able to shake out of the keyboard and wrap the keyboard with a plastic nylon covering such as saran wrap in order so no Chametz touches his hands during Pesach. To note, that while the widespread custom is to be lenient to allow typing on a computer during Chol Hamoed, some are stringent.

Explanation: Although there is no halachic requirement to clean one’s keyboard for Chametz if he recites Kol Chamira [i.e. Bittul] before Pesach even though it is most certainly filled with crumbs, being that the crumbs are dirty and unreachable and less than a Kezayis, nonetheless, practically one should not use it, and perhaps it is even forbidden to use it during Pesach, being that one may come to touch Chametz and get crumbs on his fingers which will later end up in his food or mouth, heaven forbid.

Sources: Regarding the letter of the law exemption from cleaning it from Chametz see: Admur 433:19; Regarding the prohibition to touch Chametz, see: Admur 440:16; 444:13; 446:7; 450:24-25; Rivash 401; Biur Halacha 446 “Biveiso”