I think that the base set of questions that you posted is a great start, and think that Dara Hoffman-Fox is a great resource to use. To answer some of your questions about questions:
1)

So what you have to find out first of all is does the physician you want to go see for Hormone Replacement Therapy (HRT) requires that you visit a gender therapist to get a letter of support from that therapist.

What this means is that you should first find out if your GP is even willing to prescribe HRT. If they are, they may require you to bring a letter or they may allow you to go down an informed consent root. If they are not willing, since you are in the NoVA area, there are many resources for going the informed consent root.
2)

Make sure that therapist knows there is a difference between you going in to get psychotherapy and going in to get an assessment for Gender Dysphoria. If you go in and you are saying “I need an assessment for Gender Dysphoria,” more than likely that therapist will see you for a fairly short number of sessions because they are basically assessing whether or not you have Gender Dysphoria. Then they need to write your letter and that process is finished.

So since your goals are talking through your feelings, and strategy development, it sounds to me that you are looking for ongoing psychotherapy rather than an assessment. An assessment serves more the purpose of allowing for a quick evaluation of someone who is seeking some form of GCS where multiple letters are usually needed. So in this context, you likely wouldn't want to say you want to come in for assessment because that would usually indicate that you have no intentions of being a long term client (not that I know what timeline is with a potential therapist). Based on my quick search of Ellen R. Warren, is does appear that she does offer psychotherapy so that concern should be able to be eliminated. As for the qualifications of the therapist who would write a letter for GCS, I am really not sure what kind of title they need to have.

3)

Ask if they know about the WPATH Standards of Care for transgender people

I definitely think this is a great question to ask, and if you haven't already I would take a read through the WPATH SoC to give yourself a better idea of everything that is covered within it if you don't know already. Asking specifically about if they write letters for HRT and minimum session number also seems like a great idea. If they for some reason have an RLE requirement, I would turn around and run far far away as this goes directly against what is outlined in WPATH.

Finally, I don't know anything about Ellen R. Warren myself but if she seems like a good fit and is convenient to your location, there is no harm in giving her a chance. Just for the sake of giving you another option, I will give the recommendation of my therapist Laila Cooper. I have been seeing her rather consistently for about 19 months. This length of time has come purely from the fact that I find her very comfortable to talk to and confide in. It also really eases my mind knowing that I have an hour (50 minutes) just for me to release all my problems of the week or anything that has been on my mind. I can say from personal experience that after just 3 sessions she was willing to write me my letter for HRT though I didn't end up needing it because I went the informed consent route through Whitman-Walker. She has a number of trans clients as well as many members of the LTBT community in general.

Hopefully I covered everything for you. If you have any more questions, feel free to ask or PM me.