Since you can log into your account I assume you have a computer and access. Sign up for a free Skype account and purchase 10 minutes of time for $10 USD and call Marriott using Skype at 2¢/min. This is a great way to stay in touch with friends and family when traveling abroad. Even a mobile phone call shouldn't be much more than than a $1-2/min. OK, enough on the telecom lesson.

Since you said your wife is Plat, I assume your reservation is for 4 people and was made under your wife's name.

You should be fine as your wife's reservation was made and her certificate applied prior to the hotel upgrade from 4 to 5. But yes, you should call Marriott to confirm. Here is a link to their worldwide contact info: https://www.marriott.com/rewards/customer-support.mi

Breakfast is provided in the lounge Monday-Friday at US and Canada Marriott Properties per Marriott Rewards Terms and Conditions. So if your stay is for a Sunday-Thursday night then yes, you can get breakfast in the lounge. Note: A few US and Canada hotels go above and beyond and provide breakfast on weekends even though they are not required to by the T&C so also check the Sticky at the top of the Marriott Rewards forum titled MASTER THREAD: Domestic & International Executive Lounges Open/Closed on Weekend for more info about the LAX hotel at which you are staying.

Your wife should get an email from the hotel a week before your arrival date with a link to the virtual concierge. Just have her click on that link to send an email to the hotel.

Just to be clear since you are using "I" throughout your post, I assume you mean "we". If not, then I sense there is some underlying info missing from your questions that could have a bearing on the answers above.

For example if the reservation was made under your name, then the benefits you receive will be based on your status. To change that, when you check-in you will need to have your wife show her Plat card to receive lounge access. You may even want to put her name and MR number on the reservation in order to get the 50% bonus points. Upgrades are often done in advance based on status, but I guess with 4 to a room you are looking at 2 queens and since that is what I often reserve when traveling with friends it drastically reduces the possibility of a significant upgrade (other than a CL room). Good luck.