I know the feeling... I have had times where it seemed I just could not afford the cost of a real expert. I see Andrew has offered to help - you could not be in better hands, He is very knowledgeable.
But looking at the issue of falling ranking... The URL re-write and sitemap.xml are not the main reason you have lost ranking. We have some sites that have never used URL re-write, and on some sites we generate a xml sitemap using one of the many online services that do this for free. (https://www.xml-sitemaps.com/). Our sites get good rankings regardless.
Since 21 April 2015, Google has been giving much more attention to these areas:
1, Mobile Responsiveness - sites that are not compliant with mobile efficiency factors are starting to drop down the rankings - not because Google drops them down, but because your competitors are Improving their sites faster than you are. They are "leap-frogging" you and this is the major cause of steady falls in SE ranking.
2. Pagespeed. The faster your site loads, the better the search engines (and customers) like it. Faster sites will get more favourable ranking than slower sites.
3. Structured data markup. While this is not yet having a major impact on results ranking, the use of structured data makes your content more "understandable" to google and it will know how to place your links into search results that are more relevant to the search terms a person uses. In many cases, it may appear you are climbing the rankings, but the reality is that competitors are falling lower - you are now doing the leap-frogging.
To me, these three issues are amongst the more important, so we focus on getting the site's software to comply with what is now expected by Google.
Sure... get the sitemap and URL issue sorted out - but the real issues with good ranking are in 1, 2, and 3 above.
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