Does having lead-in lights in sight allow descent below MDA?
No, you may not descend below MDA having only the lead-in lights in sight.
Reference FAA Order 6850.2b, &quot;Visual Guidance Lighting Systems&quot;, page 1-3 through 1-5.
According to this order, the list of approach lighting systems is as follows:

Medium Intensity Approach Lighting System (MALS).

Medium Intensity Approach Lighting System with Sequenced Flashers (MALSF).

Medium Intensity Approach Lighting System with Runway Alignment Indicator Lights (MALSR).

Simplified Short Approach Lighting System with Runway Alignment Indicator
Lights (SSALR).

High Intensity ALS with Sequenced Flashers (ALSF-2).

Omnidirectional Approach Lighting System (ODALS).


The Lead-In Light System is not in this list.  Instead, it is found further down the page.  Here's the key sentence:

The LDIN may be terminated at any approved approach lighting system, or it may be terminated at a distance from the threshold compatible with authorized visibility minimums permitting visual reference to the runway environment.

The LDIN is not an approach lighting system and so you may not use it to descend below MDA.