1) That happens in some cases. It is sort of a probationary period. Assuming that you have a brand new domain with little to no backlinks and suddenly acquire 5k backlinks from a 301 redirect, it will trigger redflags. Normally, organic backlinks don't happen overnight...they trickle in. It is no cause for alarm if your backlink profile is solid. It will come back up.
2) There a few factors that determine the speed, reputation of backlink sources and domain age being a couple of them. If someone with an aged domain plagiarizes your content, they initially get the benefit of the doubt. Blackhatters are notorious for buying aged domain names, scraping contents and creating PBNs with them for link building purposes. You also have to look out for negative SEO. The variables and possible scenarios are way too many to list here. Results may even vary from industry to industry. The more competitive the industry and keywords are, the harder it is to rank well.
3) What I have observed from personal experience has more to do with the balance. For instance, if you have a CF 40 with a TF 12, it's a bit questionable. It means majority of your backlinks may be spammy. A site with a CF 20 and a TF 19 will easily outrank you. The issue I have found with DA is that it often disregards spam. You can easily increase your DA by throwing 2M spammy backlinks at it from Fiverr services or with a tool like Scrapebox. If you buy a domain name with a high DA, you should still scrutinize the backlinks. Another factor is the relationship of the backlinks to your website topic. Assuming I have a fitness website that suddenly acquires a ton of accounting related backlinks to my homepage, it will trigger redflags all over the place.
When you buy domains to redirect, keep in mind that the backlinks of the domains will have to be maintained. As they fall off, your rankings will steadily drop. You will have to keep buying more domains with good backlink profiles to counter that.