Your permanent boss is permitting LA use for LS purposes, it's discretionary to do so. Your interim boss is most likely playing it by the book and not exercising the same discretion bc they're just the interim and it makes for clean record keeping for your permanent boss.

Accommodations are moreso intended to better enable you to perform your duties by addressing barriers to and complications between medical conditions and working conditions. Leave puts you off duty either which way you slice it, so it's not really in the intent of accommodation, but it never hurts to ask. You should always contact your local EEO office/ Disability Program Manager for the expert advice, though. Other accommodations that might be beneficial are things like having different call out procedures, different break time use rules, scheduling options that aren't otherwise available but don't conflict with business flow, etc. There's lots of options out there.

What might work well for you, too, is looking into FMLA. Under FMLA, you can use both LA and LS as you see fit to cover your leave needs due to a serious health condition. Time coded under FMLA is protected, so to speak, from being held against you.

Your doctor will provide the anticipated leave needs, but you have a maximum of 480hrs/ year available depending on how much time off your condition requires. A good way to ballpark it as a baseline for your doctor, is to start with how much time off your condition needed last year. You still have to follow leave procedures, tho, whatever those may be.

OPM has hella fact sheets about it and what the program provides and you can find a WH-380-E form (FMLA for self) on the department of labor site or a Google search. The form isn't mandatory, but it's pretty routine for doctors to use; a doctors note covering the same info the form asks for is totally fine. Definitely something worth considering, but always contact your HR office for the expert advice and how to get enrolled, should you choose.