Author of The Best Damn Web Marketing Checklist, Period! Active in web marketing for almost 20 years. President of Pole Position Marketing, a successful web marketing firm specializing in web presence optimization, including: SEO, Social Strategy, Website Architecture, and Keyword research.
In my own experience, I have found that simply asking often works wonders. Maybe not immediately, but follow up several months later. Be sure to include a link that will take them directly to the review area.
That said, the answer depends a lot on the type of business you are in and it might make sense to pay a digital marketer to help you develop and implement a customer review strategy.
I think the best way to do that would be to sit down with them first to find out what they think is or isn't working and to discuss ways to improve the office. If you can get them involved in the change process you'll get a lot more buy-in. They may even come up with similar or better ideas.
But be sure they understand that while you're allowing it to be a collaborative process, it's you who has to make the decisions and not all decisions will be popular, but they are what's best for everyone in the long run.
I would consider every product page on your site a landing page worth driving traffic to. Almost everything you would do to optimize a landing page for performance can be done to your internal site pages. This will 1) help you get better results from your existing pages and 2) save you time from having to optimize multiple pages for the same thing.
Successful SEO depends on two things: The amount of time the SEO has to invest (which often correlates to your budget) and the skills of the SEO doing the work. Without those two things, it's difficult to answer question number one.
SEO, and digital marketing, in general, require a lot of skillsets that most people don't have. They have some, but not all. They should, however, be letting you know what activities they are doing and explaining why they are important and what the results should be.
As for the second question, there is a lot to do and it's a matter of prioritizing. Ten SEOs will tackle the same project ten different ways, but that doesn't necessarily make any of them wrong. A great resource you can use to understand what an SEO can/should be doing is webmarketingchecklist.com (disclosure: I wrote it.) This will at least give you a place to start an intelligent conversation and set priorities.
As for milestones, you need to determine what your goals are. Are you going for sales, leads, traffic or conversions? What are you getting now and what do you hope to achieve. Then have a conversation with your SEO to determine if your expectations are realistic with the time they have to invest.
I would always choose sub-folders over sub-domains. For the most part, sub-domains are treated as separate websites (not entirely, but for the most part), and sub-folders are treated as part the main site. Links to internal pages tend to pass more weight than links to external sites.
Since you have experience managing the account, you have a general idea of how much time you spend on it. Hiring an expert may or may not need more hours, than what you have been investing, but that is something you can suss out in a conversation and after they review the account.
Once you know how much time they'll spend then you just need to determine an amount that offers them fair value for their time based on their level of experience. You can either choose to pay them by the hour or you can do a percentage model.
As an agency, we charge a percentage of ad spend. In general, every thousand dollars of ad spend requires an hour's worth of work. This is non-exact as it also depends on how much testing and landing page optimization is being done, but it's a good rule of thumb.
The percentage, however gives us incentive to keep the spend profitable and ensure the client sees a solid ROI. The better the ROI the more they are likely to spend, which increases our income and gives us more hours to optimize the campaign for better performance.
The trick is to never increase your budget if you're not getting good ROI out of it.
Of course, campaigns that start on small budgets are trickier because they need time and data in order to be optimized. If your ad spend is low then consider paying hourly until the campaign is improved enough to increase your spend and move to a percentage model.
Here's an article on pay-for-performance models: http://marketingland.com/love-pay-performance-seo-wont-heres-209555. It focuses on SEO, but many of the same principles apply to PPC.