, there is no specific timeline for it, as long as the main applicant is on the correct visa/extension. BUT I don't think there is an embassy/consulate that would issue a Non-O visa for a dependent of a Non-O retirement holder.
You seem to be mixing some information. What you are referring to is the conditions for applying for a 1-year extension after obtaining an in-country visa for a 90-day initial stamp.
For an initial in-country visa application, there is no condition about maximum remaining days with your current stamp, while there is one about minimum remaining days, 15 days or 21 days, depending on the local immigration office.
You can apply for an in-country visa even on the next day you arrive, as long as you meet the requirements.
As a rule, monthly income method for a 1-year extension based on marriage to a Thai national is one of:
1. Embassy letter/affidavit for monthly income
2. Monthly income based on working legally in Thailand.
Monthly international transfers is not one of the measures that are stated.
Some offices accept monthly international transfers of minimum 40K baht every month with proof of source of funds. But as it is not the stated method, immigration office can say no.
You should talk with your local immigration office for details and whether they would accept it for your specific case.
You may need to cancel the extension based on employment to apply for an extension based on marriage to a Thai, regardless of whether you get a new Non-O or not. In that case, you need to cancel the extension officially by using the letter from your ex-employer at the immigration office that issued the extension, it'd be the Chaengwattana office in your case, no other office.
Just leaving Thailand and coming back is not enough for this.
, KL is known for a rather long processing time. You need to check with them. I think I saw the post about that Non-immigrant visa processing time in KL is a bit shorter than DTV/tourist visa, but I cannot assure.
, employer needs to prepare more documents for you to apply for an initial visa at the local immigration office than for the application to RTE/RTCG. Also, some employers don't meet the requirements for in-country initial visa procedures. That's why you need to talk with the employer and cannot choose on your own.
No minimum number of stay-out is specified to enter Thailand again. You may be able to enter Thailand on the same day you cancel your extension if you just return to Thailand.
BUT, if you apply for your new visa at the RTE/RTCG in another country, you will stay there until the visa is issued.
You seem to mix the procedures a bit. You will take one of these after you cancel the current extension, then leave within the cancellation stamp's validity.
1. Return to Thailand on a visa-exempt for a 60-day stamp. Then go to the local immigration office to apply for an initial Non-B visa.
OR
2. Apply for the initial Non-B visa to the RTE/RTCG during your stay in that country. You wait there until the Non-B visa is issued. Then will enter Thailand on a newly issued visa for a 90-day stamp.
You need to talk with your employer about which one you must follow. It's not your choice. It depends on the documents your employer prepared.
depending on what method op uses to meet financial requirements, but proving international transfers for funds could be an issue to apply for an in-country initial visa if he wants to use 800k baht banked money method.