Most likely if you've been told you have the maximum total exposure with Chase that they are willing to give, that will stay the same unless there is a change in income and/or assets. Not sure I understand the lowering limits part of your post. You can definitely lower limits with Chase, which some do preemptively before applying for a new card in hopes of an approval or a higher starting line. If you are asking if lowering your limit to $500 will affect future approvals from other lenders that is going to be a no as long as the rest of your profile supports higher limits. I have an Amex Blue that I lowered to $500 and never use (it's been years) but haven't brought myself to closing it because it's 20 years old; that has certainly not impacted my starting lines.
As far as Marriott goes, you can certainly downgrade to the $95 AF version from Brilliant whether you have had it in the past or not, you just can't apply for it outright as the agreement between Amex, Chase, and Marriott, was that Amex would issue new premium and business cards and Chase would issue the remaining new consumer cards. Both lenders still service their previous overlapping products and both still allow product changes between them, i.e. you can PC Brilliant to the lower tier Amex card or PC any of the Chase consumer versions to the higher tier Ritz-Carlton card from JP Morgan.
Marriott has many brands, so pricing is all over the board. From Fairfield Inn and Element up to Ritz-Carlton and St. Regis, with 26 other brands in between. Anyone consistently staying in Marriott properties a few times each year should easily be able to recoup the annual fee of the Brilliant, as the $300 credit applies to any room charges, including the stay itself, and you would have to try very hard to get a redemption less than the remaining $150 for the 50k certificate (I am averaging around $750 in value for that certificate each year as I use it for a "splurge" stay). Plus the Gold elite level that comes with the card is useful for those who do not otherwise have it as while the perks offered to Gold elites may be minimal the earnings on stays are elevated, which offers additional value toward future stays.
