You should be aware there are people here that claim to be experts on gaining weight but they themselves aren't overweight.
I would be more apt to follow what a real overweight person or an active gainer suggests than someone that has no real world experience.

I would suggest eating more at each meal.
You can snack in between meals too.
I was very skinny most of my life and hated it.
I had no curves to speak of and was super embarrassed about it.

Egos are a hard thing to deal with.

I was on Dims for years and found some really nice people there. I left when there was some kind of format change and I couldn't log back in.
Went to feabie when it first went active and only stayed for a month because it was like being back in high school.
All the cliques and really mean younger girls and their snarky attitudes made it unbearable.
All of the guys acted like they were between 12 and 18 wanting naked pics to fap to.
Most were really rude and demanding so I left.
I knew if I stayed I would get banned telling some 18 year old mod to kiss my butt.

I have been here for a good 13 years or so and I never got into the pay for play scenario to me it seemed kind of sleazy at the time.
The more I stayed I understood why some girls / web models would do that and it was for income so I couldn't knock them for doing that.
Its never been something I was interested in doing because I had a career already.
I have found guys don't read women's profiles and that gets to be a problem.
They assume you have a fetish or kink and want to do nothing but fat talk.
I tell them go read my profile it says I am not here for a hook up and I don't have any fetishes.
I am here to make friends as in people to talk with kind of friends.
I came here from a different fat acceptance site because some GFs on there asked me to come here and hang out with them.
I am just here to hang out with other fat folks.

Weight gain is a slow process so 1 to 2 weeks will net you no measurable results.

If she is a" real" long time friend I think she would understand.
If not then she isn't really a friend that will accept you for being you.
I lost a few friends over my weight mostly guy friends tho'.
I have some women friends that constantly talk about being fat and it always feels weird because I am the fat one and they are not fat at all.
I would send the email and just be done with it.
You said your piece and she knows how you feel. Keeping things all bottled up and stressing over it is just not worth it.

Too many variables to suggest a defined number.

Question 1.
Its really hard to say gaining 25 lbs may be harder than you think but in my case I noticed the first 10 lbs I gained.Others if they did notice never said anything.
I could always tell when I put on weight by my underarms getting thicker and fat on my thighs getting thicker and rubbing.
Question 2.
Being bloated by eating more than what you are used to is normal.
Question 3.
Eat whatever you want just larger amounts.
Question 4.
Lets say you do gain 25 lbs yes you will probably have to go to a larger size in pants and shirts.
Question 5.
Eat whatever you want just more of it.
The is no real magic food to make you gain weight.
Ice cream and heavy cream have lots of calories so having some before bed time is a way to get your gaining started.
Eating calories then going to bed and not exerting any energy will help you gain.