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I'm analyzing a client's website today and I found that they have over 300 spammy sites linking to a subdomain of their main site. So for example, say their site is clientsite.com, well they have hundreds of links pointing to deadsite.clientsite.com. That subdomain was used at one time as a staging site, and is no longer active. Are those hundreds of spammy sites hurting or potentially hurting my client's SEO? Or is it a non-issue because the links point to a dead subdomain? We believe that that staging sub domain site was hacked at one time, and thats where all those spammy links came from. Should I disavow them?

My site was hacked some time ago. I've since then redesigned it and obviously removed all the injection spam. Now I see in search console that I'm getting hundreds of url errors (from the spam links that no longer work). How do I remove them from the search console. The only option I see is "mark as fixed", but obviously they are not "fixed", rather removed. I've already uploaded a new sitemap and fetched the site, as well as submitted a reconsideration request that has been approved.

I recently redesigned my website (studio35design.com), and with that, I'm finally paying attention to SEO. I used a few different tools to find the amount of backlinking domains I've built up through the 7 or so years. The numbers are the following: 52 Total Backlinking Domains (800+ links):
• 17 of those are from blog commenting - they are pretty much all NF
• 20 of those are from sitewide footer credits on websites I've built. They are all old, as I stopped adding sitewide links to my client sites over a year ago. 
• 7 of those are from spam. My site was hacked last year which contributed to alot of it. I've since then completely scrapped my old site and built this one on webflow (Not self-hosted like my old WP site).
• The other 10 or so are from forums, and the odd photo credit. As you can tell, almost none of these backlinks links are really any good. I'm surprised that my DA is 27 really. So moving forward, I need to know the best strategy dealing with these "bad" links. For the spam, I'm going to disavow in google webmasters. As for the rest, here are my questions... QUESTION 1:
As for the sitewide links, would the best thing be to remove the link altogether from the footer of each one of those sites? Some I can't because they are really old, and I don't have access to them (I'll probably disavow), but the rest I can still go in and update. QUESTION 2: 
Will removing a ton of links at once hurt my DA and my google ranking? I can probably eliminate 500-600 links within a few days just by removing the sitewide links. How quickly can I expect those links to drop off? A few months? QUESTIONS 3:
Should I disavow blog commenting links even though they are NF? Does it matter at all? QUESTION 4:
I noticed 5 or so of the domains I have links one aren't working domains anymore. How quickly can I expect those links to drop off? Thanks in advance for your help.

I launched my redesigned website 4 days ago. I submitted a new site map, as well as submitted it to index in search console (google webmasters). I see that when I google my site, My new open graph settings are coming up correct. Still, a lot of my old site pages are definitely still indexed within google. How long will it take for google to drop off or "de-index" my old pages? Due to the way I restructured my website, a lot of the items are no longer available on my site. This is on purpose. I'm a graphic designer, and with the new change, I removed many old portfolio items, as well as any references to web design since I will no longer offering that service. My site is the following:
http://studio35design.com