Need tips for a timid dog / dog intros

Ok, so I've been all pictures and no talk. Today I am all talk, no pictures. Miss Shelby is settling in as best as a scared dog possibly can. That's where I need YOUR help.

I don't know if Shelby has lived inside a home, but she is potty trained. On night #1 there were no treats interesting enough to get her to venture outside with. She held it all night long. The next morning, I physically had to drag her every few feet, praise and love her, repeat, until we moved past our big bad pool to get to the yard (which was out of sight due to the layout of our yard). Once she saw the grass, she was thrilled and got down to business. Once done, she darted past the big bad pool, shaking, tail tucked, to the back door waiting to get back to the safety of our master bedroom. Last night and this morning it was easier to get her to the yard, but probably because the hard food switch and treats gave her the big D. Amazing that she held it like she did all day and most of the evening.

She likes to lay under my desk or in the safety of the ex pen we set up when we realized that asking her to climb up stairs to a plastic airline crate just wasn't a good choice for her right now. She will explore parts of the house with me, but shakes and tail tucks quite a bit. She and Bellona have met through the ex pen and have shared the same room while awake and sleeping while safely seperated by the ex pen. I have been praising and rewarding them both for good behavior. Bellona however likes to talk a lot and her talking is very growly. I try to redirect her and or have her move when she does that because I can't tell if it is further stressing Shelby out.

Last night, the babysitter who comes over so that I can study 2x a week opened the baby gate I had up keeping Shelby in the room with me and Bellona in the rest of the house. I didn't hear her do this (on the phone with the rescue) and turned around to see Bellona investiagating the contents of the ex pen. Shelby was under my chair and didn't move an inch and made no sounds which is a good thing- I think. I ushered Bellona out just to be cautious. I like dog intros slow and steady. Which gets me to thinking, if I can't get Shelby out of the bedroom, let alone the house, how can I intro them on neutral turf like I typically would after a long walk together? Add to that Shelby is HW+ and I don't want to over extert her or stress her out.

I'd love any tips you have for easing a soft and timid dog into a home. I am being patient with her but would love any insight on bringing her out of her shell and integrating her into our home so we can make her a wonderful family pet for someone. I am so used to pushy, dominant, stubborn dogs that I feel a bit lost with this sweet soul. I really appreciate the help my blogger friends.