From the included photos, the holly appears to be exhibiting normal growth. Leaf drop in spring on many evergreens is typical, as older leaves are shed as new growth emerges. If this seems more extensive than in past years, then perhaps the plants experienced more stress during the past two years or so, and as such, have more old foliage to lose to balance-out its root health and new leaves. Evergreen leaves have a limited lifespan, though they are certainly relatively long-lived, the interior and lower leaves are among the oldest that have reached the end of their usefulness to the tree.

The potential for the neighbor's downspout to flood this area during overly-wet weather is a concern, so shunting it further away may be beneficial. However, if the water has room to spread out without soaking the holly excessively, then it may not be an issue. Perhaps the neighbor would be interested in creating a rain garden on their property to help corral the water, or installing rain collection barrels. Rebates for qualifying projects are available through your county: https://www.montgomerycountymd.gov/water/rainscapes/index.html.

The use of Ironite, Holly-tone, and fertilizer spikes are likely excessive with regards to the needs of this long-established plant. Although you shouldn't worry about trying to reverse this at this point, you likely won't need any more supplements for the rest of this year or beyond. Mature landscape plants generally don't need extra nutrients - especially if the lawn in the area gets fertilized regularly - and over-fertilizing can contribute to polluting runoff and pest or disease outbreaks that take advantage of over-nourished plants. Similarly, symptoms of plant malnourishment can present themselves when nutrient deficiencies aren't actually the cause; often, it is an issue with root health or soil conditions (like pH) instead. We don't see any reason to suspect such causes here, though it's possible last year's late-season drought caused some minor root stress for the holly.

Miri